Here are a few of my favorite fanfic links, mostly related to Hercules, Xena and Buffy. This is very nowhere near complete, of course -- I'll be adding more links in more fandoms with every update.
Other People's Stories
Hercules and Xena (Slash)
Jen's Iphicles Slash
Odysseys and Ecstasy -- Thamiris' stories.
Wild: Narcissus' Slash
Buffy and Angel (Gen and Slash)
Eterniata -- excellent multi-author archive
Blood and Chocolate -- Sarah T.'s fanfic
Sandra McDonald's site
Harry Potter (Gen and Slash)
FictionAlley -- More HP fanfic than you can shake a broomstick at.