M.U.G.E.N. is a 2D fighting game engine that is easy to modify to include custom characters, stages and other features.
There are thousands of custom characters and customized versions of MUGEN, so if you’ve ever wanted to square off with Sponge Bob vs. Homer Simpson in a fighting game, this is the one for you.
The game is free to download, but it looks like the company behind it, Elecbyte, is AWOL since 2013.
Fortunately there is a huge community around M.U.G.E.N. at MUGEN Archive.