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.
Even tho it is aging and it is beginning to show its weakness comparred to other more modern engines, it is still my most preferred engine and the only one I was able to get into succesfully. And with all of the updates Valve is giving it, I think the source engine will be with us for a long time.
I wouldn't really recommend this for a commerical product, just because there are so many more modern engines out there that will be a lot easier in the long run (like UDK), but all of the commercial games that I have seen on the source engine are fantastic (like Bloody Good Time ect.)
The Source engine is a must for just about any mod, it is very easy to change and manipulate the engine, there are a **** ton of tutorials floating around the internet, and also because if its good enough, steamworks!
Long Live the Source Engine.