The Old One, cosmic entity, many tentacled, star of designer board games and dark cultists everywhere. Created by H. P. Lovecraft, this monster has come to life far more than Lovecraft ever could have envisioned. There are other Old Ones in the Lovecraft universe, to be sure, but it is Cthulhu that reigns supreme.
The Cthulhu influence can be felt in everything from comic books (Mike Mignolia’s Hellboy), games (Eldritch Horror) and pulps (Robert E. Howard’s in particular). Strange that there have not been more movies featuring Lovecraft’s monster. The first Hellboy movie touched on the Old One, and I am certain that if Guillermo Del Toro’s At the Mountains of Madness ever gets made, it will be a worthy introduction to the Lovecraft mythos to the masses. On second thought, perhaps that is the Old One’s plan all along.
The role-playing game Call of Cthulhu is still popular in RPG circles. References to Cthulhu can be found in the old Real Ghostbusters cartoon as well as the Teenage Mutant Ninja Turtles and Scooby-Doo cartoons. A handful of songs credit the same Old One as inspiration as well.
I would also like to mention a particular short story by Neil Gaiman called A Study in Emerald that mixes Sherlock Holmes and Cthulhu together into a madness inducing cocktail of Victorian alternate lore.
Pretty prolific for a slimy centuries old entity from beyond known space and time. No wonder he is adored by dark robed cultists everywhere.