Mugen is a free 2D fighting game engine. It allows you to create your own fighting game (with your own characters, backgrounds...). Mugen is very configurable and powerfull, you can get a gameplay as good as commercial fighting games such as Street Fighter Alpha or King of Fighters series. The only weak point of Mugen is the difficulty to create characters. But the more difficult is to begin, after some work it becomes easier, and you'll love it :). But if you don't have time or patience to make your own characters, you needn't worry: you can download characters already made by other people.
"Mugen" is a word of Japanese origin, meaning dream, fantasy, or infinite... "M.U.G.E.N" is a freeware 2D fighting game engine designed by Elecbyte, released on July 17th, 1999. The program was written in C with the Allegro library. Beta versions were made for DOS, Linux and Windows, and was distributed through their website. "M.U.G.E.N" is actually an unknown acronym. The creators, Elecbyte, never told what it was.
Basically, M.U.G.E.N allows users to create their own 2D fighting game, using the core game engine provided. From the characters, to the stages, to the background music, to the life bars and power bars, text font, and even in-game character stories... You can create pretty much anything you want once you learn the programming, and it's not incredibly hard either. If you ever had a dream of creating your own fighting game, M.U.G.E.N is a good "practice run," it is good .