Yuletide is a massive, Secret Santa-style fanfiction exchange that has run every Christmas since 2003. Its focus is on obscure, old, or unpopular fandoms. The assumption is that the people who are writing fanfiction about Harry Potter or Star Wars have plenty of outlets for their writing and large potential audiences, while those who want to read Allo Allo or John Steinbeck fanfiction don't get a chance to find it very often.
What qualifies as "obscure" changes from year to year. Some new movies or series take off quickly, giving them only one or two Yuletide seasons before being retired; others which may have had an active fandom many years beforehand might be revived in Yuletide as the majority of their writers move on to other, trendier projects. Historical and AU fiction always have a place at Yuletide; some of it hardly feels like "fanfiction" at all, since it more closely resembles original fiction set in another author's setting or starring real-world figures.
The Yuletide process is quite ingenious. Sometime in October, potential participants are invited to suggest the fandoms they would like to see on the list. (The list gets weirder every year, and makes for some fascinating reading.) The requests are compiled and character lists are populated with participant input. Then, in November, the final list is put online at http://www.yuletidetreasure.org.
At this point, anyone who wants to -- unless, that is, they defaulted in a previous year -- can sign up to write something. You must request either three or four fandoms, and you must offer to write at least three (up to as many as you like). After about a week of signups, a computer matches readers to writers, and assignments are sent out. Any readers who cannot be matched get their requests publicized for "pinch hitters," that is, people who volunteer to write an extra story for one of the orphans. Nothing is finalized until everybody gets a story.
Stories need to be submitted by December 21. (There are some allowances for withdrawal, as long as there is time to find a pinch hitter; if you withdraw too late, or without notifying anyone, you are banned from future Yuletides.)
The stories -- thousands of them -- are posted to the website, anonymously, on Christmas Day. On New Year's Day, the authors' names are revealed. Writers who want to keep going after that can do what they call "New Year's Resolution" stories to fill out the database; usually they choose one of the requests that wasn't filled by the assigned writer (remember, every reader had three requests).
I neither write nor read 'fanfiction' strictly defined, but I like worldbuilding and I enjoy historical fiction. I also love experimenting with writing styles and adopting the "voice" of writers I admire. So even though I'm personally not that interested in writing porn about Dexter and his sister or whatever, I still think Yuletide is pretty ace -- it gives me a chance to explore new ideas and provides an inescapable deadline so that I can get to writing. (Not entirely unlike the current e2 quest, come to think of it.)
The clever programming automates the whole thing through the Yuletide web page. In my short experience, it runs surprisingly smoothly, given that there are nearly 2,000 participants and at least as many stories to keep track of. The community, which generally congregates on Livejournal, is friendly and easy-going. Even if you're not a part of "fandom", it's worth taking part.
http://www.yuletidetreasure.org