Night diving... deserves a quiet night. And a bundle, since Night Dive Studios are responsible for porting plenty of scary old classics to PC, and their wares are now available on the cheap courtesy of the latest Humble Weekly Bundle.
Specifically, you can beat £2.95 for Wizardry 6 and 7, Shadow Man, I Have No Mouth and I Must Scream, Harvester & the 7th Guest. An interesting mix of vintage RPGs, hammy-yet-creepy FMV puzzlers and a truly dark point & click adventure.
Paying £5.31 will also net you Wizardry 8, System Shock 2 & The 11th Hour - which is worth it if you don't already own System Shock 2. It's still one of the most terrifying and effective horror games ever made, and a great hard sci-fi FPSRPG that directly inspired the BioShock series.