Most customization options in a fighting game

The freeware PC-based brawler M.U.G.E.N. treads a fine line between game and development tool. It allows gamers to customize almost every aspect of its 2D fighting mechanics, and a continually active fan community has produced enormous ammounts of user-generated content, including unique arenas and fighters as well as legally dubious appearances by characters from TV, movies and other games. Guinness World Records Gamers' Edition 2009.