M.U.G.E.N
From Game Making Tools Wiki
MUGEN is perhaps a slightly odd fit, in that it is not so much a game creation tool, as it is a framework for created gighting game characters (and other assets). That's not to say that sometimes packages aren't released containing a setup of characters and stages, and treated as a game in its own right.
See Also
There are lots of MUGEN sites, here's a selection.
- MUGEN Fighters Guild - Big MUGEN community.
- MUGEN Archive - Big download site.