The Quake engine is the game engine that was written to power 1996's Quake, written by id Software. It featured true 3D realtime rendering and is now licensed under the GPL. After release it immediately forked, as did the level design. Much of the engine remained in Quake II and Quake III Arena. The Quake engine, like the Doom engine, used Binary space partitioning (BSP). The Quake engine also used Gouraud shading for moving objects, and a static lightmap for nonmoving objects.
Concidering countless spin-offs, iterations, re-writes, branches, and most importantly the fact that it aged so damn well, one can easy say that Quake Engine is for game engines exactly that, what Unix is for operating systems. Or something like that. I'm definitely onto something here... :)