Half Life 2, Episode 1, Episode 2 and Lost Coast have just released on linux in full native support. Left 4 Dead 2 as also just hit beta on the linux platform. Those who own Left 4 Dead 2 should find the beta in their Steam Library and be able to play on Linux. Furthermore the beta offers access to the Extended Mutation System, authoring tools and dedicated servers. This means that the Linux build is now up to the same level as the PC and Mac versions.
In the most recent update to the beta, cross platform play between all three OS’s is now supported.