Half-Life

From CSWiki

Half-Life, often abbreviated as HL or HL1, is a sci-fi first-person shooter computer game developed by Valve Software, first released by Sierra Studios on November 19, 1998. The game uses a heavily modified version of the Quake engine, called GoldSrc.

There were a lot of mods released for half-life, including CS, it later became a stand-alone game, but still needs to install some half-life files to function.