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.
Despite it's age, the source Engine is an ever-expanding, fully functional game engine with many perks and capabilities that I feel make it an excellent choice for Game Developers and Modders. While it lacks some of the qualities that other Engines posses (such as DX10+ Support and Large-Scale worlds), Valve's continued efforts to expand upon the Engine to insure that it remains compatible and glitch-free are certainly one of the many reasons why for 6 years and counting, people from all across the globe are playing and modding the games blessed with the power of Source.
I hope Valve never lets go of this one. Great graphics, sound, effects, and online support.