Here you can find links to public source codes of famous applications and games.
Electronic Arts DeluxePaint
http://www.computerhistory.org/atchm/electronic-arts-deluxepaint-early-source-code/
Mozilla BrowserQuest
https://github.com/mozilla/BrowserQuest
Doom 3 BFG Edition
https://github.com/id-Software/DOOM-3-BFG
Star Wars Jedi Knight: Outcast and Academy
Originally hosted on sourceforge but taken down soon after.
https://github.com/grayj/Jedi-Academy
https://github.com/grayj/Jedi-Outcast
Adobe Photoshop 1.0.1
http://www.computerhistory.org/atchm/adobe-photoshop-source-code/