EDIT: You're kidding right? The Half-Life series is listed in there. I know for a fact that both the Source engine and Goldsrc engine contain legacy code from the quake engine, which is an upgraded version of the doom engine, created by the same company.
Hahahaha, you are retarded. The Half-Life series comes from Valve, *not* id. Half-life 2 uses the Source engine, which was written from scratch by Valve.
Half-life 1 (and spinoffs) did use the GoldSrc engine which was a heavily modified Quake engine. But nobody uses the original Quake engine anymore. The last game to use the Quake 3 engine was Call of Duty, and even then it was extremely modified.
But even then, only 21 games used the Id Tech 3 (usually referred to as Quake 3) engine. http://en.wikipedia.org/wiki/Id_Tech_3#Uses_of_the_engine
The Doom 3 engine was written from scratch as well, and only 6 games have used the Id Tech 4 engine.
If anyone here seriously thinks that modern, shader driven DirectX 9.0c and DX 10 games still use code from even 8 years ago is an idiot.
This is one case where I do know what I'm talking about. The source engine was not coded from scratch. It heavily relies on the legacy code from the goldsrc engine, which is a modification of the quakeworld engine.
I did not say that they used the engine directly, but that most games (especially in the fps genre) use legacy code from the original id Tech. (Engine behind doom and quake.)