On November 23, 2011 id Software maintained the tradition and released the source code of their previous engine. This time is was the turn of idTech4 which powered Prey, Quake 4 and of course Doom 3. Within hours the GitHub repository was forked more than 400 times and people started to look at the game internal mechanisms/port the engine on other platforms. I also jumped on it and promptly completed the Mac OS X Intel version which John Carmack kindly advertised.
In terms of clarity and comments this is the best code release from id Software after Doom iPhone codebase (which is more recent and hence better commented). I highly recommend everybody to read, build and experiment with it.
Check out his entire code review here.