The Left 4 Dead 2 Beta has been merged with the standard game giving Linux support and the tools to script your own mutation modes. Mutations are my favorite part of the game as it lets you mix up the gameplay in various ways (ie. M60's with unlimited ammo or swords only). This has already lead to some cool stuff, like a game mode where all the zombies are about as big as smurfs. Lots of cool community content on the way.

More info:
http://www.pcgamer.com/2013/07/03/left-4-dead-2-update-brings-linux-support-adds-extended-mutations-to-workshop/