CallOfCthulhu is, as you all know, a correctly spelled* horror RPG published by Chaosium and based on the writings of H.P. Lovecraft and his hangers on. We're using the 5th edition, with a few HouseRules, and probably a load of background material from 4th ed. and earlier, as they have a lot of nice 1920s stuff in them.

*I initially spelled Cthulhu Cthulu; thanks to Ben for pointing this out before there was too much to change... -- RobHague