Eldritch, the Lovecraftian first-person roguelike, has now had its source code released to the public. You can get your hands on it for free.
All you have to do is go to this part of the official site and follow the links.
Eldritch is developed by a pair of former AAA first-person shooter veterans in the form of David Pittman (BioShock 2 and XCOM Declassified) and his brother Kyle Pittman (Borderlands 1 and 2). So it has some serious pedigree.