Cytadela is a free and open-source old school 3D first-person-shooter game. The game was originally released in the early 1990's for the Commodore Amiga and is a cross between Wolfenstein 3D and Doom. Cytadela has been rewritten and converted to OpenGL.
The Citadel is a secret military research base located on a distant planet. After the contact with the base has been lost, a military spaceship has been sent there with a mission to either restore the communication or, if that's impossible, destroy the base. While approaching the planet, the spaceship has been destroyed by the Citadel's defence systems and you, as the only survivor, have no other option but to fulfil the mission on your own - you have to destroy the Citadel. In order to do that you have to penetrate the whole base, find six parts of a bomb and plant it in the Command Centre. Your task is by no means simple - there are hordes of enemies who will try to stop you, and you will have to solve many riddles in order to find your way through the base.
Mac OS X 10.4 Tiger or newer
Intel or PowerPC processor
OpenGL compliant graphics card
Free download at sourceforge.net