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.
Really poorly updated, the Hammer Editor is the same as 1999 and the SDK have a lot of important bugs who are still not fixed. Most of the tools are made by the community and the other officials are extremly rudimentary....