This is a freeware fighting came for Windows PCs. It is unique in that it lets you add in your own fighters and stages, and music, and life/power bars, and in general edit almost every aspect of almost every screen in the game's user interface. You can make your own characters, stages and addons for it, or download those made by others that they've posted on their own sites. There's many community forums dedicated to this game.
You can get the latest version of the game from the creator's official website http://elecbyte.com/mugen
You can get a character editor/creator freeware called Fighter Factory Ultimate from this website http://www.virtualltek.com/downloads
FFU is a GUI program, and the best character creator software I've seen, and beats the heck out of the official character creation command-line programs that were made by Elecbyte (the company that made the actual M.U.G.E.N. game).