Goldsource

The Goldsource engine is the engine that Half-Life 1 and all of its expansions uses. It is based on both the Quake engine and Quake 2 engine. It uses QuakeWorld as a base and has some of the improvements that Quake 2 added pulled over. Several games other than Half-Life use this engine, such as Counter-Strike, Gunman Chronicles, and Nightfire.