GoldSrc

GoldSrc (also known as GoldSource, and formerly just the Half-Life Engine) is a 3D video game engine created by Valve in 1996. It was the driving force behind many famous games of the late 90s and early 00s, such as Half-Life, Team Fortress Classic, Counter-Strike, and Day of Defeat. GoldSrc is a heavily modified version of the Quake engine and runs on C++ (with some "C" code). GoldSrc and its level editor, Valve Hammer Editor, were released by Valve for public use, making it the source of countless community-made modifications.