Source, the game engine developed by Valve, just got an update to its software development kit to support Linux:
We have released an update to the Source SDK, bringing support for Mac OS X and Linux to mod developers and exposing the ability for virtual reality support in your mod. The biggest change with this update is that we are using github to host the source code. You will find the code here. This Source SDK 2013 release also includes a new license that can be found here. This new license allows mod authors to share their changes to the SDK more easily.
The other change with the Source SDK is that now Hammer and the other mod tools ship with their respective games instead of as part of the SDK Launcher. The launcher itself is being phased out, so it will disappear from your Tools list. You can find information about how to run the tools from the games here.
The source for this new SDK release includes the latest code for all the included games, and has many new features:
• The games now build and run clients on Windows, OSX, and Linux. Dedicated servers are supported on Windows and Linux.
• Steam Pipe (the new Steam content delivery system) is supported by the sample mods. Existing mods can change their gameinfo.txt to match the new format and gain Steam Pipe support.
• Support for Virtual Reality via the Oculus Rift has been added to the SDK. Running a compatible mod with -vr on the command line will run the mod in stereo and enable head tracking on the Rift.
You can find instructions on getting started with the new Source SDK 2013 on the Valve Developer Community wiki.
A large number of modifications use the Source engine already.