I’m not a huge fan of the DOOM and Quake series of games myself, but I will admit the engines are pretty good and one thing that’s very cool about iD Software is that they like to let the source code to previous engines loose into the wild after a few years and a couple of new engines are developed. This is very cool because there are a lot of talented amateur coders out there who can learn a lot from iD’s example and perhaps come up with the next uber game mod ala Counter-Strike. In addition to this it allows folks to port the game to other platforms it may not have appeared on previously such as Linux or OS/2 or even the venerable Amiga. For example there’s a website devoted to all the platforms you can find the original DOOM ported to called It Plays Doom that gives a great idea of how far folks can run with access to the source code. So a big kudos to iD Software for keeping the tradition going.
You can get the source code to Quake 3 from here among many other places.