A source port is a software project based on the source code of a computer game's engine that allows the game to be played on operating systems or computing platforms with which the game was not originally compatible. Source ports are created by fans after the original developer hands over the maintenance support for a game by releasing its source code to the public. The term was coined after the release of the source code to Doom. Due to copyright issues concerning the sound library used by original DOS version, id Software released only the source code to the Linux version of the game. Since the majority of Doom players were DOS users the first step for a fan project was to port the Linux source code to DOS. A legitimate source port includes only the engine portion of the game and require that the data files of the game in question already be present on users' systems.
(taken from Wikipedia)
On this website you will find alist of many source ports from very well known games.
2011-11-15 Doom source code
From John Carmack's Twitter account:
"doom 3 source is packaged and tested, we are waiting on final lawyer clearance for release."