Brought to fruition by Elecbyte in 1999, Mugen is a scripting program that emulates the functions of a standard, 2D fighting game. And not unlike commercial fighters, Mugen boasts various modes of play with comparable degrees of action. It allows for complete customization (with characters, backgrounds, sound effects, fonts, etc) of a desired layout. From a creative standpoint, the engine's flexibility gives one the freedom to code those aforementioned elements however he/she deems fit. Generally speaking, dream matches and fighting game crossovers are a reality with Mugen.
To this day, the precise meaning of the engine's abbreviated moniker remains a mystery, but it's said to roughly translate to the term "infinity". The original developers are seldom about, but the community is kept alive by devoted players and contributors. Here are several screenshots of Mugen in all its glory.