For those that don’t know. ID software is an games development company and subsidiary of zenimax media (same guys who publish Bethesda me thinks).
There most notable projects are probably Doom, Quake and Wolfenstein. There most recent creation "Rage" wasn’t a huge hit but still worthy of mention.
Well up until now, They’ve had a notable developer to port their creations over to the Linux OS’s (big linux fan here). Recently though he seems to have jumped ship but thats besides the point.
While most of their older games source code has been available for a while now, It can be a bit of a mess trying to find/organize it. Now (in what appears to be a response of him leaving), a lot of games have been hosted in a single location and open sourced.
So the point of this entire post? If you wanna see how some of the pros do coding in the industry (albeit a bit outdated), nows your chance.