Wolfenstein
The "Wolfenstein" series of video games started with the 1981 stealth-adventure video game "Castle Wolfenstein". It was developed by American programmer Silas Warner, a pioneer in the early eras of video gaming, especially the stealth genre. It was published by his (then) company M.U.S.E. Inc. (Muse Software). The player controls an unnamed American prisoner of war as he steals war files containing information about secret German war plans while avoiding or sometimes killing guards, and tries to escape the fortified Nazi stronghold Castle Wolfenstein, set during World War II. The game is often credited as one of the first video games of the stealth genre, since it focuses more on avoiding or disarming enemies, and killing them is considered a last resort.
Wolfenstein: Enemy Territory
The game uses a modified "Return to Castle Wolfenstein" engine, itself being a heavily modified id Tech 3 engine, which has been open source since 2005. As of the first day of the August 2010 QuakeCon, the entire source code was released under the GNU General Public License v3.