Steam just released the Source SDK for 2013, adding support to Macintosh OS X and Linux, as well as support for Oculus Rift, thereby introducing Virtual Reality to the engine.
As a quick refresher, Source is a video game engine introduced by Valve in 2004 with the game Counter-Strike: Source. What's unusual about it is they have been updating the engine in small increments lacking a numbered updating scheme. Valve has released an SDK for Source they also update infrequently for the benefit of developers to mod their existing games.
Interestingly enough, Valve says the biggest change they've made with this update is hosting it via popular software developer resource host github. Although Source and Valve games remain proprietary, for all intents and purposes, the Source SDK has made Valve an active participant in the modding and remix community. These changes expand the capabilities of Source modders and developers. Oculus Rift support alone enables developers to start programming with the device for free, preempting Oculus Rift's own consumer SDK.
We reported that Valve decided to start releasing the Source SDK for free two years ago.
Image is from Garry's Mod.