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 one of the most powerful video game engines of all time!, and also has one of the best graphics of all time. And also one of the best video game titles has been made with this wonderful engine: Half Life 2, Grey, Nightmare House 2, Black Snow and most importantly, Black Mesa, the best video game made with the Source Engine. And in conclusion this video game engine was made by one of the the best video game companies of all time, Valve Corporation.
My favorite commercial engine. It performs nicely on my subnotebook with an integrated Intel HD 4000. While usually 3D engine ages so very fast the constant updates make those awesome Valve games still look good on todays standard. Well crafted and a sane business decision to keep games constantly updated rather than creating boring sequels which is the video game industry standard. The Steam integration is so well done, it makes me forget that I don't really own the game, it is full of DRM and my subscription can be canceled any time.
Back in 2004 when it was released, people didn't really know much about game physics or High-Dynamic Range, so it was the engine of that generation. It was, and it still is very flexible, user-friendly and has been used to make tenths of games and mods, such as Team Fortress 2, Portal, Left 4 Dead, Counter-Strike: Global Offensive and Garry's Mod, and of course many other mods. Now Valve is left behind popular companies like Crytek, Activision and Ubisoft, but once Source Engine 2 is released, it will be on top of the other games.
Source is an extremely versatile and powerful engine, and Valve have done some very impressive things with it. However, the tools for it, especially Hammer, are very outdated and not user-friendly. On the upside, you are given the source for HL2 with no sandboxing, which is certainly an advantage over Unity, where you are forced to use sandboxed scripts. However, the user-unfriendly tools make it difficult to quickly prototype or start a game in the Source Engine.