From Black Mesa: Wiki

Valve's Source Engine is a game engine which has been used to power all Valve games since the release of Half-Life 2 and Counter-Strike: Source in 2004. Source is recognizable for it's distinct feature set including radiosity-based lighting, support for detailed facial animation, detailed physics simulation, and it's scalable rendering (DirectX 7-9). Engine updates for Source have included various next-gen features including High Dynamic Range rendering, particle system support, and integrated support in the content creation pipeline for the Xbox and Xbox 360.

The Source engine is notable for it's large amounts of modifications, which can be attributed to Valve's support for mod creators, by providing the SDK, a developer wiki, and distributing an easy method for running and distributing modifications with the Source SDK bases. In addition to mods, a number of commercial studios have also taken advantage of the Source engine to produce games; see the list on Wikipedia.