If you've ever wanted to create your very own Street Figter game, look no further than M.U.G.E.N.
According to Wikipedia:
"M.U.G.E.N is a freeware 2D fighting game engine designed by Elecbyte, written in C with the Allegro library.
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 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."
The video below demonstrates with a battle between Ronald McDonald and Doraemon.