I asked something among the lines of this question and I'm going to leave the best of what I got here:
What are the most useful gems to use in Rails?
Abstraction
ActiveInteraction - Manage application specific business logic.
Cells - View Components for Rails
Interactor - Interactor provides a common interface for performing complex interactions in a single request
Light Service - Series of Actions with an emphasis on simplicity.
Mutations - Compose your business logic into commands that sanitize and validate input
Reform - Form objects decoupled from models.
Admin Interface
ActiveAdmin - a Ruby on Rails framework for creating elegant backends for website administration
bhf - A simple to use Rails-Engine-Gem that offers an admin interface for trusted user
RailsAdmin - A Rails engine that provides an easy-to-use interface for managing your data
Analytics
Ahoy - A solid foundation to track visits and events in Ruby, JavaScript, and native apps
Gabba - Simple way to send server-side notifications to Google Analytics
Legato - Model analytics reports and queries against the official Google Analytics Reporting API
Staccato - Track analytics into the official Google Analytics Collection API
API Builder
ActiveModel::Serializers - JSON serialization of objects
Crêpe - The thin API stack
Grape - An opinionated micro-framework for creating REST-like APIs in Ruby
jbuilder - Create JSON structures via a Builder-style DSL
JSONAPI::Resources - JSONAPI::Resources, or "JR", provides a framework for developing a server that complies with the JSON API specification.
Jsonite - A tiny, HAL-compliant JSON presenter for your APIs
Pliny - Opinionated template Sinatra app for writing excellent APIs in Ruby
rabl - General ruby templating with json, bson, xml, plist and msgpack support
Rails::API - Rails for API only applications
Roar - Resource-Oriented Architectures in Ruby
Assets
Less Rails - The dynamic stylesheet language for the Rails asset pipeline.
Less - Leaner CSS, in your browser or Ruby.
Sass - Sass makes CSS fun again
Management:
Rails Assets - Bundler to Bower proxy
Sprockets - Rack-based asset packaging system
Authentication and OAuth
Authlogic
Clearance - Small and simple email & password based authenticaton for Rails
Devise - A flexible authentication solution for Rails based on Warden
OmniAuth - A library that standardizes multi-provider authentication utilizing Rack middleware
Sorcery - Magical Authentication for Rails 3 and 4
OAuth:
Doorkeeper - An OAuth2 provider for Rails
OAuth2 - A Ruby wrapper for the OAuth 2.0 protocol
Authorization
Authority ORM-neutral way to authorize actions in your Rails app.
CanCanCan
Pundit - Minimal authorization through OO design and pure Ruby classes
Caching
Action caching for Action Pack - Action caching for Action Pack
Dalli - A high performance pure Ruby client for accessing memcached servers
Record Cache - Cache Active Model Records in Rails 3
CLI Builder
Commander - The complete solution for Ruby command-line executables
GLI - Git-Like Interface Command Line Parser
Main - A class factory and DSL for generating command line programs real quick
Rake - A make-like build utility for Ruby
Slop - Simple Lightweight Option Parsing
Thor - A toolkit for building powerful command-line interfaces
CMS
Alchemy CMS - A powerful, userfriendly and flexible Open Source Rails CMS
LocomotiveCMS - A simple but powerful CMS based on Liquid templates and Mongodb database
Publify - A self hosted Web publishing platform on Rails
Radiant - A no-fluff, open source content management system designed for small teams
Refinery CMS - An open source Ruby on Rails content management system for Rails 3 and 4
Code Analysis and Metrics
Brakeman - A static analysis security vulnerability scanner for Ruby on Rails applications.
Flay - Flay analyzes code for structural similarities. Differences in literal values, variable, class, method names, whitespace, programming style, braces vs do/end, etc are all ignored. Making this totally rad.
Flog - Flog reports the most tortured code in an easy to read pain report. The higher the score, the more pain the code is in.
fukuzatsu - Complexity analysis tool with a rich web front-end.
MetricFu - A fist full of code metrics
rails_best_practices - A code metric tool for rails projects
Reek - Code smell detector for Ruby
Rubocop - A static code analyzer, based on the community Ruby style guide.
Rubycritic - A Ruby code quality reporter.
SimpleCov - Code coverage for Ruby 1.9+ with a powerful configuration library and automatic merging of coverage across test suites.
Coding Style Guides
Rails style guide - Community-driven Rails best practices and style for Rails 3 and 4
RSpec style guide - Better Specs { rspec guidelines with ruby }
Ruby style guide - Community-driven Ruby coding style
Concurrency
Celluloid - Actor-based concurrent object framework for Ruby
Concurrent Ruby - Modern concurrency tools including agents, futures, promises, thread pools, supervisors, and more. Inspired by Erlang, Clojure, Scala, Go, Java, JavaScript, and classic concurrency patterns.
EventMachine - An event-driven I/O and lightweight concurrency library for Ruby
Configuration
Configatron - Simple and feature rich configuration system for Ruby apps
Configus - Helps you easily manage environment specific settings
dotenv - Loads environment variables from .env
Econfig - Flexible configuration for Rails applications
Figaro - Simple, Heroku-friendly Rails app configuration using ENV and a single YAML file
Global - Provides accessor methods for your configuration data
RailsConfig - Multi-environment yaml settings for Rails3
Core Extensions
ActiveSupport - A collection of utility classes and standard library extensions.
Ruby Facets - The premiere collection of general purpose method extensions and standard additions for Ruby.
Attributes
ActiveAttr - What ActiveModel left out
FastAttributes - FastAttributes adds attributes with their types to the class
Virtus - Attributes on Steroids for Plain Old Ruby Objects
Hash
Hashie - A collection of tools that extend Hashes and make them more useful
Country Data
Carmen - A repository of geographic regions
Countries - All sorts of useful information about every country packaged as pretty little country objects
i18n_data - country/language names and 2-letter-code pairs, in 85 languages, for country/language i18n
normalize_country - Convert country names and codes to a standard, includes a conversion program for XMLs, CSVs and DBs
Dashboards
Dashing-Rails - The exceptionally handsome dashboard framework for Rails.
Data Visualization
RailRoady - Ruby on Rails 3/4 model and controller UML class diagram generator.
Rails Erd - Generate Entity-Relationship Diagrams for Rails applications.
Ruby/GraphViz - Ruby interface to the GraphViz graphing tool
Database Drivers
Cassandra Driver - A pure ruby driver for Apache Cassandra with asynchronous io and configurable load balancing, reconnection and retry policies
DataObjects - An attempt to rewrite existing Ruby database drivers to conform to one, standard interface.
mongo-ruby-driver - MongoDB Ruby driver
moped - A MongoDB driver for Ruby
mysql2 - A modern, simple and very fast Mysql library for Ruby (binding to libmysql)
Redic - Lightweight Redis Client
redis-rb - A Ruby client that tries to match Redis' API one-to-one, while still providing an idiomatic interface
ruby-pg - Ruby interface to PostgreSQL 8.3 and later
SQLite3
Database Tools
Database Cleaner - Database Cleaner is a set of strategies for cleaning your database in Ruby.
PgHero - Postgres insights made easy
Seed dump - Rails 4 task to dump (parts) of your database to db/seeds.rb.
Seed Fu - Advanced seed data handling for Rails.
Date and Time Processing
business_time - Support for doing time math in business hours and days
Chronic - A natural language date/time parser written in pure Ruby
groupdate - The simplest way to group temporal data in ActiveRecord, arrays and hashes
time-lord - Adds extra functionality to the time class
time_diff - Calculates the difference between two time
validates_timeliness - Date and time validation plugin for ActiveModel and Rails
yymmdd - Tiny DSL for idiomatic date parsing and formatting
Debugging Tools
Byebug - A simple to use, feature rich debugger for Ruby 2.
debugger - A port of ruby-debug that works on 1.9.2 and 1.9.3.
Decorators
Draper - Draper adds an object-oriented layer of presentation logic to your Rails application
DevOps Tools
Backup - Provides an elegant DSL in Ruby for performing backups on UNIX-like systems
Capistrano - A remote server automation and deployment tool written in Ruby
Chef - A systems integration framework, built to bring the benefits of configuration management to your entire infrastructure
Lita - ChatOps for Ruby: A pluggable chat bot framework usable with any chat service.
Logstash - Logs/event transport, processing, management, search
Mina - Really fast deployer and server automation tool.
Puppet - An automated administrative engine for your Linux, Unix, and Windows systems, performs administrative tasks (such as adding users, installing packages, and updating server configurations) based on a centralized specification
Rubber - The rubber plugin enables relatively complex multi-instance deployments of RubyOnRails applications to Amazon's Elastic Compute Cloud (EC2).
Vagrant - Create and configure lightweight, reproducible, and portable development environments
Documentation
Asciidoctor - A fast, Ruby-based text processor & publishing toolchain for converting AsciiDoc to HTML5, DocBook, EPUB3, PDF & more.
grape-swagger - Add swagger compliant documentation to your Grape API
Inch - Inch is a documentation measurement and evalutation tool for Ruby code, based on YARD
RDoc - RDoc produces HTML and command-line documentation for Ruby projects
YARD - YARD enables the user to generate consistent, usable documentation that can be exported to a number of formats very easily
E-Commerce and Payments
Active Merchant - A simple payment abstraction library extracted from Shopify
Paypal Merchant SDK - Official Paypal Merchant SDK for Ruby
Piggybak - Modular, Extensible open-source ecommerce solution for Ruby on Rails
ROR Ecommerce
Shoppe - A Rails-based e-commerce platform which allows you to easily introduce a catalogue-based store into your Rails 4 applications
Spree
stripe-ruby - Stripe Ruby bindings
Ebook
Bookshop - Bookshop is a an open-source agile book development and publishing framework for authors, editors.
Eeepub - EeePub is a Ruby ePub generator.
Gepub - A generic EPUB library for Ruby : supports EPUB 3
Git Scribe - Basically the best way to write an ebook.
Mobi - A Ruby way to read MOBI format metadata
Review - Re:VIEW is flexible document format/conversion system
Email
Incoming - Incoming! helps you receive email in your Rack apps
LetterOpener - Preview mail in the browser instead of sending.
Mail - A Really Ruby Mail Library
MailCatcher - Catches mail and serves it through a dream
Mailman - An incoming mail processing microframework in Ruby
Pony - The express way to send mail from Ruby
Environment Management
chgems - Chroot for RubyGems
chruby - Change your current Ruby. No shims, no crazy options or features, ~90 LOC
fry - Simple ruby version manager for fish
gem_home - A tool for changing your $GEM_HOME
rbenv - Use rbenv to pick a Ruby version for your application and guarantee that your development environment matches production
ruby-build - Compile and install Ruby
ruby-install - Installs Ruby, JRuby, Rubinius, MagLev or MRuby
RVM - RVM is a command-line tool which allows you to easily install, manage, and work with multiple ruby environments from interpreters to sets of gems
Error Handling
Airbrake - The official Airbrake library for Ruby on Rails (and other Rack based frameworks)
Better Errors - Better error page for Rack apps
Errbit - The open source, self-hosted error catcher
Exception Notification - A set of notifiers for sending notifications when errors occur in a Rack/Rails application
Nesty - Nested exceptions for Ruby
Raven Ruby - Raven is a Ruby client for Sentry.
File Upload
CarrierWave - Classier solution for file uploads for Rails, Sinatra and other Ruby web frameworks
DragonFly - A Ruby gem for on-the-fly processing - suitable for image uploading in Rails, Sinatra and much more!
PaperClip - Easy file attachment management for ActiveRecord
rack-secure-upload - Upload files securely
Form Builder
Abracadabra - The gem that swaps out text with a fully-compliant Rails form in one click
Formtastic - A Rails form builder plugin with semantically rich and accessible markup
Rails Bootstrap Forms - Rails form builder that makes it super easy to create beautiful-looking forms with Twitter Bootstrap 3+
Simple Form - Rails forms made easy
Game Development
Gosu - A 2D game development library for the Ruby and C++ programming languages
Yeah - Practical Ruby video game framework
Geolocation
Geocoder - A complete geocoding solution for Ruby. With Rails it adds geocoding (by street or IP address), reverse geocoding (find street address based on given coordinates), and distance queries
Geokit - Geokit gem provides geocoding and distance/heading calculations
Git Tools
git-auto-bisect - Find the commit that broke master
git-spelunk - Dig through git blame history
git-up - Fetch and rebase all locally-tracked remote branches
git-whence - Find which merge a commit came from
hub - a command line tool that wraps Git in order to extend it with extra features and commands that make working with GitHub easier
Rugged - Ruby bindings to libgit2
GUI
QtRuby - Qt4 binding for ruby
RubyGnome2
Shoes
HTML/XML Parsing
HappyMapper - Object to XML mapping library, using Nokogiri
Nokogiri - An HTML, XML, SAX, and Reader parser with XPath and CSS selector support
ROXML - Custom mapping and bidirectional marshalling between Ruby and XML using annotation-style class methods, via Nokogiri or LibXML.
HTTP
excon - Usable, fast, simple Ruby HTTP 1.1. It works great as a general HTTP(s) client and is particularly well suited to usage in API clients.
Faraday
Http Client - Gives something like the functionality of libwww-perl (LWP) in Ruby
http - The HTTP Gem: a simple Ruby DSL for making HTTP requests.
httparty
Patron - Patron is a Ruby HTTP client library based on libcurl.
RESTClient - Simple HTTP and REST client for Ruby, inspired by microframework syntax for specifying actions
Savon - Savon is a SOAP client for the Ruby programming language.
Sawyer - Secret user agent of HTTP, built on top of Faraday
Typhoeus - Typhoeus wraps libcurl in order to make fast and reliable requests
Imagery
MiniMagick - A ruby wrapper for ImageMagick or GraphicsMagick command line
PSD.rb - Parse Photoshop files in Ruby with ease
RMagick - RMagick is an interface between Ruby and ImageMagick
Skeptick - Skeptick is an all-purpose DSL for building and running ImageMagick commands.
Internationalization
i18n-tasks - Manage missing and unused translations with the awesome power of static analysis
i18n - Ruby Internationalization and localization solution
r18n - Advanced i18n library for Rails, Sinatra, desktop apps, models, works well with complex languages like Russian.
twitter-cldr-rb - Ruby implementation of the ICU (International Components for Unicode) that uses the Common Locale Data Repository to format dates, plurals, and more
Logging
Cabin - Structured+contextual logging experiments in Ruby.
Fluentd - Fluentd collects events from various data sources and writes them to files, database or other types of storages
HttpLog - Log outgoing HTTP requests.
Log4r - Log4r is a comprehensive and flexible logging library for use in Ruby programs
Logging - A flexible logging library for use in Ruby programs based on the design of Java's log4j library.
Lograge - An attempt to tame Rails' default policy to log everything.
MongoDB Logger - MongoDB logger for Rails
Scrolls - Simple logging
Yell - Your Extensible Logging Library
Machine Learning
PredictionIO Ruby SDK - The PredictionIO Ruby SDK provides a convenient API to quickly record your users' behavior and retrieve personalized predictions for them
Ruby Datumbox Wrapper - It's a simple Ruby Datumbox Wrapper. At the moment the API currently allows you to build applications that make use of machine learning algorithms.
Markdown Processors
kramdown - Kramdown is yet-another-markdown-pars
er but fast, pure Ruby, using a strict syntax definition and supporting several common extensions
Maruku - A pure-Ruby Markdown-superset interpreter
Redcarpet - A fast, safe and extensible Markdown to (X)HTML parser
Misc
AASM - A library for adding finite state machines to Ruby classes
AXLSX - An excel xlsx generation library
Betty - Friendly English-like interface for your command line. Don't remember a command? Ask Betty
Foreman - Manage Procfile-based applications</li