Source SDK, Valve's free software development kit, has been updated to include support for Mac OS X and Linux mods, as well as Oculus Rift.
Other improvements to the SDK include the ability to build and run clients for Windows, OSX and Linux. According to Valve's post, dedicated servers are now supported on Windows and Linux. Additionally, Hammer and other mod tools will now ship with their appropriate titles as part of the dev kit's launcher.
The source code is now being hosted via github. The full code is available here, while the dev kit's new license, which allows mod authors to share their changes, is available here.
Valve added Oculus Rift support for both Team Fortress 2 and Half-Life 2 earlier this year.