Fanfiction.net is the largest single archive of Fanfiction on the internet, covering fandoms based on anime and manga, books, TV, movies, plays, musicals, comics and games.
As of January 2011, ff.net hosted over 44,000 Supernatural fanfics, 225 communities and 74 discussion forums relating to Superantural.
The communities generally act as places to gather fics with a particular theme. The most active discussion forums are those hosting role playing games.
Since September 12, 2002 (Source), the site disallowed NC-17 rated fic. There is a separate site called adultfanfiction.net for fic with this rating, but it is little used with less than 500 Supernatural fics archived there as of December 2009.
RPF is also disallowed on the site.
See also Rec Communities