Call of Cthulhu
Call of Cthulu is a RPG setting based off of the World of H.P. Lovecraft. At 25 years old, this original system is in its fifth edition.  For more information prodeed to the Chaosium Website.
Rumors lurk that Cthulhu will soon be available in a D20 format. Cthulhu: D20 is due out in March, 2002.