DCSteve AIMed me the following very exciting news:
Just a few minutes ago id Software released the source code for Quake III Arena!
If that would be compiled with KallistiOS for Dreamcast, the scene might expierence a revolution in legal independent gaming.
You can download the source code for Q3A v1.32b here.
Older versions, free demos, add-ons and shareware releases of Quake are available here.