Valve has successfully completed testing the Linux version of Left 4 Dead 2 along with Steampipe support and the Extended Mutation System, with all three now available as the latest official build.
As detailed on the Left 4 Dead blog, the Extended Mutation System is a tool for modders that allows creative players to build new Mutations and game modes and add them to Left 4 Dead 2's Steam Workshop.
Valve has promised to highlight its favourites and add them to official servers, but for now it recommends Holdout, a multi-map Mutation with resources and crafting.
The other major update in the latest patch is SteamPipe integration, which doesn't mean much on the user end except faster patching.
Finally, the Linux version is now live on Steam and open to any interested users with supported distributions.
For more information, you can check out the full patch notes. To grab the update if it hasn't already installed, just restart your Steam client.