Killer 2D Fighting game. Fully editable, load your own characters (or those from any fighting game in fighting game history) effects, soundtrack, customize backgrounds, oh what the hell... The whole game.
Check out Youtube for some cool custom Mugen's. I've been working on one for a few years now. Thing got pretty big, around 6GB for the entire game I'm doing. Used to set it up on two laptops at the subway on daniel and watch drunk show downs. It was pretty popular (till the fun police ruined everything)
Just Google Mugen Characters, Stages, or Screen Packs to see others conversions. I've got some hella rare characters if anyone gets into this and wants to do some trades.
Wiki
http://en.wikipedia.org/wiki/M.U.G.E.NDownload
http://mugen.en.softonic.com/ 