GoldSrc

GoldSrc (pronounced "Gold Source"), sometimes called the Half-Life engine, is a proprietary game engine developed by Valve. At its core, GoldSrc is a heavily modified version of id Software's Quake engine. It made its debut in 1998 with Half-Life and powered future games developed by or with oversight from Valve, including Half-Life expansions, Day of Defeat and games in the Counter-Strike series.