Many RTSs and strategy games boast extremely enthusiastic and creative communities who love to create their own mods for the game. Happily, many of the heavy hitters in the genre allow them to do just that, and Total War: Shogun 2 can now be numbered among them. Via "the Editor," or "TEd," Shogun 2 players can now create their own custom maps for both co-op and multiplayer battles. TEd works for both the original Shogun 2 as well as its stand-alone expansion Fall of the Samurai. 

Valve have released the official mod tools for Portal 2 on the PC via Steam.  Now anyone can get their hands on the game's inner workings, customize and create their own levels.

If you played the Dark Carnival campaign in Left 4 Dead 2, you know it ends with a rocking good time featuring the music of a certain band called the Midnight Riders.  While the Riders never appear in the game, they do have their own website, which perhaps alludes to an upcoming expansion featuring the cast of four characters.