id Software is a well-known company to most people that play such games as Dangerous Dave, Commander Keen, Wolfenstein, Doom, Heretic, HeXen etc. Some time ago, John D. Carmack made some statements at QuakeCon and promised to release the source code for Doom 3, but so far that has not happened.
ZeniMax company that presently is the owner of id Software (as well as Bethesda) has released an update for Doom Classic and Wolfenstein 3D for the iPhone. It added to them support of Retina Display and universality (now they can be played in iPhone and iPad), as well as it fixed some bugs in the updates.
The company offers an access to the current source codes for these games along with this update, so now you can make something of your own based on their development, or you can learn from the best game makers how properly make such games.