Doom 3 source code was just released after a minor rewrite by John Carmack because an algorithm he had independently invented in 2000, now known as (Carmack's Reverse) for handling shadows was patented a year or two later by Creative Labs where it was also invented. Avoiding the attention of patent lawyers is a good thing.
According to a comment on Hacker News Doom 3 is made up of 366,423 lines of C++ code, 110,991 lines of C/C++ headers and 53,520 lines of C plus a variety of other languages which will be I guess utilities and tools for buillding the release.
It's interesting and possibly worth a quick look over unless you are planning to mod it or develop a game from it. It's GPL'd so you must release your source code but you can copyright your own graphics and so don't have to release them which is what ID Software do. This is not a bad way to try and break into the games industry.
For interesting reviews of the technical aspects of source code of other games by ID Software take a look at Fabien Sanglard's website.
- A link to the Doom 3 Source Code was added to the Code Library