Flashfic. Flashfic comms are fandom-specific comms where new prompts are given every two weeks (usually) and anybody who wants to can write a short story (usually < 1000 words) based on the prompts. The prompts are usually general things like "darkness" or "flight" or something like that. due South has a flashfic comm, so does Dresden, and SGA. I don't think TW does, although it should.

Hee--I feel like the Encyclopedia of Fandom. Feeling overwhelmed yet? *g*