Simworld is a zero-player game in which players create simulated environments and ecosystems that run with no user intervention. The environments and the creatures that inhabit them are all completely defined through user-created mods. Though there's no real goal to the game, a good challenge is to create ecosystems that are self-sustaining and can thus run indefinitely.
Note: I'm not galaxy-brained enough to do the full idea at the moment, so this is the "lite" version, containing only the bare minimum to make the idea work.
