M.U.G.E.N – Most popular multi-player 2D fighting game maker

M.U.G.E.N (AKA: Mugen, MUGEN, 無限) is a discontinued but still very active free 2D fighting game engine with many customizable components. It was developed by Elecbyte, first released in 1999 (for DOS). In 2009, MUGEN 1.0 was released, and in 2013, MUGEN 1.1 came out.