Showing my age now, but one of the first games I ever played was Cholo, which was a very early 3D sci-fi game. It was a forerunner for 3D environment games, and was well ahead of its time.
The game is set in a post-nuclear war city called Cholo, where the radiation levels have now become safe for people again. However, the human population is trapped in a bunker while the city above is guarded by robots. At the start of the game you have control over a small robot of your own. Your job it to take control of additional robots within the city and discover a way to destroy the bunker seal.
Cholo has just been remade by a team called Ovine, and released as freeware. Because it's a remake they've kept to the original robot designs, but in all other respects it has been completely updated. Nostagia value aside, it's a great little game. Try it out!