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.
This is inevitable of the biggest engine ever made. Remember, when they show to us this absolutely amazing physics, and that was a big revolution back then! Yeah, it's becoming old, and Source2 is coming, however, I think, Source 2 will never completely replace THE Source engine.
And again, it's lightning system or his cubemap system is pretty bad, now that we have real time lightning and cubemaps... But it gives it his little charm.