Left 4 Dead 2 In Beta For Linux
The Linux build of Left 4 Dead 2 is in beta right now. If you own a Steam copy of the game, you should see the beta version in your library immediately.
Taking part in the Linux beta will also give you access to the game's authoring tools and its dedicated server beta. It's not a limited beta, it's the full game. It also supports four community maps, Diescraper Redux, Urban Flight, Back to School, and City 17.
[Source: Left 4 Dead blog]