M.U.G.E.N is a freeware 2D fighting game engine developed by Elecbyte. The most recent version of M.U.G.E.N to be publicly released is version 1.1 beta 1, which was released on August 7th 2013, though the most recent stable build is version 1.0, released on January 18th 2011.
M.U.G.E.N allows users (typically known as "authors" or "creators") to create content for the engine such as (but not limited to) characters and stages for others to download and add to their game, or just to keep for themselves, as well as fully-fledged fighting games. Being a fighting game engine at heart, M.U.G.E.N takes inspiration from a variety of classic fighting game series such as Street Fighter and The King of Fighters, featuring a seven-button combat system (a, b, c, x, y, z, and start), typically representing three punches, three kicks and a taunt, though characters are not limited to having those presets, with some only having four or just three attack buttons and a taunt.
M.U.G.E.N has a fairly large community, with multiple forums and sites dedicated to the engine, as well as many internet videos with the purpose of teaching users how to use and customise the engine or (more commonly) broadcasting M.U.G.E.N fights.