This is the kind of project I've wanted to do since I was a kid but have never had the tenacity to follow through with.
Ripped from Wiki.. :
M.U.G.E.N is a freeware 2D fighting game engine designed by Elecbyte, written in C with the Allegro library. The engine was originally released in July 17, 1999. Beta versions of it were made to work on DOS, Linux and Windows platforms, distributed through their website or to donors via email. With the release of the Linux version, support for DOS ceased.
The engine allows anyone to create characters, background stages and other game objects through interpreted text files, graphics, and sound compilations. It supports various types of audio formats such as MP3 and MIDI initially, although it can be configured to play various audio formats via Winamp plugins, such as ADX and OGG, as background music during gameplay or at other points such as an introduction or the select screen. The engine allows for most of the same type of functionality found in most any commercial 2D fighting games, up to and including close recreation of those games' characters and gameplay. While the engine is set up primarily for fighting game development, several other game types have been developed using it, including shooter and platformer style games.
So you can make your own fighting games using it! Thats awesome.. Also illegal because the dubious enough programs license expired years ago and was never refreshed. But I'd love to mess about with it for a bit.