There are LOTS of fanfiction sites out there - too many to list, really. The one you're probably thinking about is
Fanfiction.net, which is a large archive for many fandoms (not just Tolkien) - but they're having server problems, and the site will be barely functional for the next two weeks (although you may be able to read some stories, you won't be able to post any or write reviews, or use the search engine). It's a pity (as well as frustrating - that's where I post my own fics!). If you haven't checked out the Fanfiction site here at the Downs, do so! Other sites you might enjoy include
Nindaiwe and
The Henneth Annun Archive Have fun reading!