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.
I love Source, it's the first and the best engine I've ever used.
The only bad thing is that you can't sell your Games without having a Special license which is be pretty expensive.