ASSTR is a huge repository of just about anything from alt.sex.stories. It may be the biggest story site on the net when you go in the front door.

http://www.bdsmlibrary.com/ is another free story site with a lot "extreme" stories.

http://www.literotica.com/ is probably the best story site on the net. Perhaps 50 to 100 new stories posted each day by authors - some very good - and almost never stories taken from elsewhere. It's not an extreme site. It's intended as an author-friendly site.