I am pretty sure that everyone reading this blog is aware of Knytt Stories, the seminal indie classic by Nifflas and one of the best exploration platformers ever. Chances are most indie gamers have already played the thing too, but, as Anna Anthropy eloquently showcased in her book, the most important Knytt Stories bit might just be its level editor. It's simple, powerful and a perfect tool to get you into level design and, up to a point, game design.
The Knytt Stories editor, you see, comes with every copy of the handily freeware Knytt Stories game and, thus, also with a built-in community and audience to help you with your design ideas and world-building skills. Oh, and using said editor is almost as easy as drawing something up in MS Paint and requires virtually no scripting.