Based on H.P. Lovecraft's classic short story of the same name, Call of Cthulhu is a classic table-top role playing game by Chaosium, that manages to capture the maddening, dark, terrifying and hopeless atmosphere of the original. It also is a rare RPG with truly simple rules, that urges you to play as a fragile professor or a decadent dilettante rather than a hairy barbarian with a huge sword. And if you don't know what an RPG is, well, have a look here, but trust me when I say that Call of Cthulhu is both an excellent game and a truly great read, and that I've yet to meet someone who appreciates horror that hasn't enjoyed his or her encounter with this sort of game.
Anyway. Follow this link and download the free 20-page introductory PDF to Call of Cthulhu. The booklet contains everything you need to start playing (besides friends and dice, that is), as it provides all the basic rules, character generation guidelines, a brief overview of the Cthulhu universe and an introductory adventure.