X-Men vs. Street Fighter(エックスメンVS.ストリートファイター) is a fighting game originally released as a coin-operated arcade game in 1996. It is Capcom's third fighting game to feature Marvel Comics characters and the first game to match them against their own, with characters from Marvel's X-Men franchise being matched against the cast from the Street Fighter series.
M.U.G.E.N (also known simply as MUGEN) is a freeware 2D fighting game engine designed by Elecbyte, written in C which originally used the Allegro library. The latest versions of the engine now use the SDL library.