Developer: Rogue Software
Release Date: May 15, 1996
Source Code Release Date: December 12, 2014
Demonstrating the flexibility of the DOOM engine (or at least developers' willingness to try and use it everywhere), Strife is an RPG with FPS elements. If nothing else, it's a neat idea. One interesting side note is that the original source code for this game was lost, the support for this game was reverse engineered.