Glass Onion
An archive for the mailing list of the same name. Consistently high quality; not much that's long, but all of it is good. Very high slash content, but a detailed labeling system. Largely defunct, apparently, but still awesome.

While We Tell of Yuletide Treasure
Home of the beloved annual rare fandom ficathon. You'll find something for fandoms you didn't know anyone else had ever heard of; quality varies widely, but it was all written with love.
The first of the huge multi-fandom archives, and always the monster that continues to spawn. I've complained about the signal-to-noise ratio a million times, but it's still a good place to start. Tread carefully, but have fun.

Twisting the Hellmouth
Buffy crossed over with everything you could possibly imagine, and a few things you wish no one had. Sometimes brilliant, sometimes horrific, always amusing.