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.
Annoying to develop for without buying some kind of guide, but runs very well when compared to other engines (although this could just be how each game is made individually) and is now capable of running on windows, mac, and linux.