MUGEN ARCHIVE

Mugen Archive (also known as The Chronicles of Mugen) is primarily a M.U.G.E.N warehouse, but also has a forum, and a chatbox at the top of the page.

Though signup isn't necessary to browse and download, it is to post.

They have a lot of different characters most places do not have, and an added bonus is they host the files on their server, which is constantly being updated.