I don't know why, but everyone can download the full Quake 4 SDK
The code seems to be complete( of course models, maps, … are missing. Examples are inside, but not the full game^^)
For anyone how doesn't know, Quake is a first person shooter.
It's really interesting and they offer a good documentation.
http://www.iddevnet.com/quake4/
Maybe someone has interest
Man I wish more people where like Mr. Carmack and more companies like ID 
He should be knighted by the entire gaming and graphics community, I like that "Sir John Carmack" 
I want an iPhone! http://www.gadgettastic.com/2008/03/10/doom-creator-looking-to-develop-id-iphone-games/
Even if I could never use it to make a call (no service here )