Slashdot just reported that the entire source code for the famed game Doom III Arena is now avialable under the GNU General Public License.
You may also want to check out the web pages of its creator - idsoftware. For those who not were there, these people are the ones who started it all by introducing the hugely popular shoot-em game - Wolfenstein. They did not stop there but went on to create the more popular Quake.
However, to day my favourite games remain to be Tomb Raider and Microsoft Age of Empires.