If you want decent Harry Potter fan fiction, most of what's on
http://www.fanficauthors.net is pretty good. The general writing quality is a lot higher than what you generally find on fanfiction.net
FFA has, to my knowledge, one slash story: Cartographer's Craft by Copperbadge.
If you're interested in (primarily) Buffy the Vampire Slayer crossover fics,
http://www.tthfanfic.org has some good stories as well.
PS: Both Fanfic Authors and Twisting The Hellmouth offer EPUB versions of their archived stories.