A.S.S.T.R.
ASSTR is home to over 1000 authors of erotic literature, host of the alt.sex.stories.moderated newsgroup, mcstories.com, mirror site for nifty.org, and host of several popular erotic literature archives.

Literotica
Literotica features 100% original sex stories from a variety of authors, on a variety of topics. A good place to go when you want to try a little of everything.

Kirsten's Archive
Very well organized and maintained collection of stories of all different types with a short description for each one before you even click the link.

Nifty Erotic Stories Archive
Erotic stories involving alternative sexualities. Probably the largest and best collection of Gay erotica in existence.