The Source engine is a 3D game engine developed by Valve Corporation. Its unique features include a large degree of modularity and flexibility, an artist-driven, shader-based renderer, accurate lip sync and facial expression technology, and a powerful, efficient and completely network-enabled physics system.
Following the relaunch of TF2 as a free-to-play game, the Source SDK and SDK Base 2006 game have been made available to everyone on Steam.
Posted by Varsity on Jun 28th, 2011
In a somewhat off-the-cuff decision (confirmed through my own testing), Valve have made the Source SDK and the shared SDK Base 2006 game free to everyone on Steam. It's slightly odd that its Source 2007 sister app isn't free as well, but who's complaining? [SDK Base 2007 is now free.]
The release coincides with a huge update to the software and visual theme for the Valve Developer Community wiki, seen below.