I tried to keep this as sort and sweet as possible, but once you're locked into a great free plugin collection the tendency is to push it as far as you can go. What follows is a list of free plugins and a short description of how I use them.
Most are mac and pc, au and vst, maybe a few other formats too unless otherwise stated, that I find useful, nay magical, just make sure your magic licence is up to date.
http://www.tokyodawn.net/tdr-nova/
Very nice dynamic eq with sidechain, handy for de-essing, or making space for instruments, sounds etc, without affecting the overall tonal balance to much like with a static eq.
http://www.tokyodawn.net/tdr-kotelnikov/
Great transparent compressor, find it useful for busses and the main output channel for just knocking a tiny amount of gain off a signal, good for mastering too.
http://www.acustica-audio.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=160&category_id=50&option=com_virtuemart&Itemid=189#key-features
Very nice character compressor, found using the attack and release settings to be different then other compressors I have, but the shmod dial helped shape the sound to my liking.
http://www.acustica-audio.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=137&category_id=50&option=com_virtuemart&Itemid=189
Another very nice character eq, sometimes I'll just use the preamp on this without the eq, just to add a subtle effect to say a drum buss.
http://www.airwindows.com/console4/
This is a great very simple plug to use, idea is after you have gain staged and added fx to your liking, plonk a channel version on the channels, or if you bus say a set of drums to an aux, put it there instead of the individual drum channels.
Then place the buss version on the master output, find it helps give a better balance and sense of spaciousness to all the individual elements of a mix, can be quite subtle, but beware though it doesn't like hot signals, so make sure to gain stage and still have headroom left (for the fx) after you've added all your fx.
The dev is in the process of porting his plugs to vst format and also windows vst and setting everything free, he has a very nice back catalog, he has switched to the patron funding model, linked below, his other free stuff is worth a look too, but most is still in au mac format.
https://www.patreon.com/airwindows
http://magnus.smartelectronix.com/#Ambience
Great reverb, nice and spacious, find it crashes in logic pro x, on el captain, but if you switch the view mode to control it works, or maybe that was in logic 9 and snowy I can't remember, but the gui was flakey on mac with au.
https://www.meldaproduction.com/download/plugins
With this you download the main installer then select the free bundle when you install. One thing I absolutely love in the free pack is the mcompressor, switch it into custom shape and experiment with different shapes on the graph, fantastic for shaping transients and general mangling.
http://finecutbodies.com/?p=sound
A nice exciter plugin, helps give some extra definition to drums, instruments, synths etc, it's called la petite excite.
http://www.anarchy-rhythms.com
This is fantastic, like it says a cross between a drum machine and fx, very creative plug, also worth checking out the other free plugins a lot of spectral stuff.
http://nickcrowlab.blogspot.co.uk/2009/08/tubedriver-v10.html
Bit of a rogue, nice if you want to fatten kick drums, bass, guitar, has a very rough sound that I find pleasing.
https://www.xferrecords.com/freeware/index.html
Great set of free plugs, I use ott for parallel compression of drums to give them that extra ooooh la la, dimension expander is useful too and the djmfilter is good for quick and dirty filter sweeps.
http://www.bozdigitallabs.com/product/width-knob/
Very simple one knob style plugin for controlling the width, although this works by narrowing the stereo image, this is very useful for example if you want a delay to bloom as its repeating, to give the impression it's expanding in the stereo field as it decays. Just automate from say 50% when the delay starts and open it out to 100% as its decaying, this also goes into negative values to flip the left and right channels. Worth checking out other freebies such as bark of dog too.
http://www.voxengo.com/group/free-vst-plugins/
Another great set of free plugs, MSED is great for setting up a m/s (mid/side) matrix, this can be useful if you don't have a m/s option on your favourite eq, compressor, saturation plug etc, but still want to use them in a m/s configuration.
For example split whatever sound you want using sends to 2 different busses/aux's (both at unity gain +0.0 in logic, no output in the original track), add MSED to both, select mode encode, then side mute for the mid channel say bus 1, then mid mute for the side channel say bus 2. Add your fx plugins then route both these channels to bus 3 on the output tab in logic not the sends tab, then place MSED on bus 3 and set it to mode decode.
Also find sound delay plugin useful for messing around with the haas fx, as it has fine tuning capabilities, although I have much to learn on this subject still.
https://developer.oculus.com/documentation/audiosdk/latest/concepts/os-vst-usage/
Info page about spatializer
https://developer.oculus.com/downloads/audio/1.0.4/Oculus_Audio_SDK_Plugins/
Download
Bit of an odd one a wild card if you like, but it looks fantastic, it works on mono sources to place sounds around a users head, the head is in the middle when you load this up. To me it sounds a bit like a binaural plugin, but turning the reflections on and the reverb makes for some nice sound experimentation. It's only vst though, works for me in maschine, which I sometimes use as a vst host in logic, by loading an fx instance of maschine.
https://vladgsound.wordpress.com/2014/08/25/limiter-no6-update-1-0-2b-and-env1-skin/
Another fantastic free plugin this is the version with my favourite skin, there is a choice of three I think on the site. This is an absolute monster of compressor/limiter/clipper/protector in one plug, fantastic on a buss for shaping or a main output for taming. Also molot is worth a download from the same site and if you work with high sample rates, this eq below is worth a look too.
https://vladgsound.wordpress.com/2014/12/21/tdr-ultrasonic-filter-alpha-version/
http://matt.eifelle.com/2015/09/15/announcement-atkcoloredcompressor-1-0-0-and-atkcoloredexpander-1-0-0/#more-2686
I like the expander in this set of two fx, adds a nice tone when I want to give transients a bit more bite or push them back a bit.
https://www.plugin-alliance.com/en/products/pa_free_bundle_v1.html
I use mainly the bx cleansweep and bx solo from this package, cleansweep is good for filter sweeps using automation and solo for expanding or collapsing the stereo image to mono, has four steps.
http://klanghelm.com/contents/products/IVGI/IVGI.php
Nice little saturation plugin that I use for adding a touch of spice, has two other free compressors that are worth checking out too.
http://www.fluxhome.com/products/freewares/bittersweet-v3
Great little transient shaper, left for decrease, right for increase also their stereo tool is worth checking out too.
Time limited stuff
https://www.plugin-alliance.com/en/products/unfiltered_audio_indent.html
Free until the 14th of august, quite useful saturation plugin, find its helps bring things forward in the mix
https://reverb.com/software/effects/acon-digital/47-reverb-solo?_aid=yt
https://reverb.com/news/exclusive-free-plugin-drift-by-sinevibes
These two you have to sign up for a reverb account, acon is a one knob reverb plug, drift I'm not sure how long the time limited offer was on for, I got it a few weeks back, worth a go though.
Honourable mentions
http://www.soundhack.com/freeware/
http://www.bluecataudio.com/Products/Bundle_FreewarePack/
http://www.auburnsounds.com/products/Panagement.html
http://aegeanmusic.com/doppler-dome-specs
http://www.alexhilton.net/A1AUDIO/index.php/downloads
http://www.digitalfishphones.com/main.php?item=2&subItem=5 (32bit osx and win)
Windows
http://www.lesliesanford.com/vst/plugins/
http://www.maxsynths.com/vst_free_sturiophonia.html
Windows 32bit
http://ugoaudio.com/index.php/vst-plugins
https://varietyofsound.wordpress.com/downloads/
http://jezwells.org/Computer_music_tools.html#VST_plugins
Mac
http://www.michaelnorris.info/software/soundmagic-spectral
I will add to this thread with any good free plugs I find on my travels, feel free to add your own gems, hopefully some of you will find this useful, altho freeware plugin junkies will know most of these.