Games with user-made content

Sometimes you'd just like to satisfy that creative urge (but not as much to leave your computer, of course). For that, level editors, modding, ROM hacks, basically any games where players can do more than just play, hit the spot. In brief, this page is for listing games with a significant community producing new content. The description should include links to such communities, as well as info on how the reader could participate (does the game include an editor, is it open-source, how conveniently it can be modded).