Beware the Machine Mother.
Released in 1998, the influence of this FPS/RPG is still being felt today, especially in the Dead Space and BioShock games (the BioShock series being led by Ken Levine who worked on this game, the gameplay between the two games are incredibly similar)
You awake from cryogenic sleep into a derelict spaceship. All around you are what remains of the crew, and the mutants that seek to end you. Guided around the ship by the malevolent A.I SHODAN, you must find a way to destroy the creatures and stop SHODAN's plans for a new world order.
The real horror of this game is not the dated visuals (although mods exist to bump up the visuals with high-res textures and models) but the audio work. A lot of gamers will be familiar with the concept of collecting audio-logs to provide back-story to the events of the game, a concept the System Shock series introduced to horror games. The taunts from SHODAN and the collective mind of the mutants are quite un-nerving, and the pleading from the mutants for you to escape before they kill you didn't get easier to listen to, even during my second play-through.
This game used to be notoriously hard to get a hold of, until it was re-released a week or so ago on GOG.com
and made compatible for modern systems. Sci-fi horror fans can consider this a must play.