Valve released the Source SDK 2013 a short time ago today and it supports Linux and OS X in addition to Windows. The Source Software Development Kit can be used for developing new game mods.
Another change with version 2013 is that the Source SDK is now hosted on GitHub. Valve has also altered its Source SDK source-code licensing so that game mod developers can more easily share their SDK modifications.
Furthermore, a small change worth pointing out is that the Oculus Rift VR solution is also supported by this SDK release.
More details on the Source SDK 2013 release can be found at SteamPowered.com.