Yuletide is a massive, small fandom, fanfic exchange. You request a story in up to six fandoms, and offer to write a story in up to ten fandoms. You're then given an assignment to write a story for someone who requested a fandom you offered, and someone else is assigned to write a story for you. Stories are posted anonymously on Christmas Day, and the writers are revealed on New Years.

It's kind of crazy, but a lot of fun.