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.
The engine uses 7 buttons for gameplay along with the directional keys, in order to accommodate six-button fighters, which use three punches, three kicks, and a start button. However, characters do not necessarily use all seven buttons, nor need to follow a traditional six- or four-button format. At most, two human players can control characters, with others controlled by the engine's AI; including a demo mode where the computer controls all characters. In addition, several gameplay modes are available via the main menu.
Source: Wikipedia, "M.U.G.E.N", available under the CC-BY-SA License.