M.U.G.E.N is a 2D fighting game engine that is enables you to create commercial-quality fighting games. Almost everything can be customized, from individual characters to stages, as well as the look and feel of the game.
The majority of content created for M.U.G.E.N tends to be distributed as individual characters, stages or motifs. Assembling a game is as simple as downloading the content of your choice, and configuring M.U.G.E.N to know about it.
M.U.G.E.N is designed to be used by people with little or no programming experience, but with some artistic talent and patience to learn. Of course, having some programming background does give you a bit of a headstart. However, if you are just looking to play with downloaded content, all you need to know is how to unzip files and edit a text file.