Although the Creation Kit mod development platform for Skyrim (previously) is still a few days from being released, intrepid modders have been working with edited game files to achieve mixed results. One of those results is the "Macho Dragon Mod" (download link). [more inside]
posted by codacorolla on Jan 9, 2012 - 81 comments

Commander Keen is series of platform games from the early 1990s, whence the Dopefish. People who grew up with DOS games will find gameplay videos and music quite nostalgic. Over the period 2006-2011, modders released a new Keen trilogy called The Universe Is Toast. Gameplay is a novel mix of eerily familiar and the flat-out weird.
posted by shii on Dec 3, 2011 - 28 comments

M.U.G.E.N [wikipedia] is a 2D fighting game engine, originally developed by Elecbyte and released in 1999. The engine is highly customizable; characters, backgrounds, and sound files -- whether from existing games or original works -- can be easily integrated. Some examples of the engine in action [youtube]: Homer Simpson vs. Peter Griffin, Ryu vs Popeye, Green Power Ranger vs Osama Bin Laden, Fat Albert vs Juggernaut, Marvin the Martian vs. Duck Dodgers. Also, game crossovers: Homer in NES Land, Thunder Force III vs. Duck Hunt. [mi] WARNING: some of the videos are very loud.
posted by milquetoast on Jun 17, 2007 - 10 comments

Front Line Force is a brand-new mod for Half-Life, and in only a few months has become the fourth-most popular Half-Life mod with approximately 150 servers (behind the undisputed king Counter-Strike, TFC, and Deathmatch).
posted by stazen on Jan 14, 2001 - 0 comments