Depends what you are after. The main sites have already been listed. A lot of sites have gone in recent years, unfortunately - for the same reason as libraries are closing. No-one reads.
Here are some more specialist sites that I personally enjoy. Might not be to your taste though...
http://www.robertlubrican.com/main.html
http://www.dangerbabecentral.com/xpa...ries/index.php
http://superheroine.tripod.com/fic/shid.htm
http://dark_mark.tripod.com/darkmark1.htm
https://mcstories.com/ (I especially recommend Downing Street (an author), ironically
https://www.fantasies.com/categories-2/ - not the greatest, but lots of categories
http://www.adult-fanfiction.org/
https://archiveofourown.org/ - more fan fiction
http://www.c-s-s-a.com/ - celebrity sex stories archive
https://www.t-s-s-a.com/stories/index.php - Television sex stories archive
http://toddcheese.tripod.com/
https://tcheser.blogspot.com/
http://www.darkwanderer.net/ - funded by pop-ups unfortunately. Stay away if you can't handle them
http://www.shokushu.com/ - it doesn't get much more specialist than this
Asstr can be difficult to navigate, as alrady notes, but has some good microsites ....
https://www.asstr.org/~Kristen/ - scroll right down to the categories and explore
https://www.asstr.org/files/Authors/Stormbringer/
https://www.asstr.org/~pervman/newsite/stories/news.htm
https://www.asstr.org/~flyingpen/penindx.htm
https://www.asstr.org/~BlackDemon/storyindex.html - interracial, with strong non-consent overtones
https://www.asstr.org/files/Collecti...storyindex.htm - Impregnorium says it all
Enjoy!