Pen-and-paper roleplaying game, published for the first time in 1981 by Chaosium. The game takes its name from H.P. Lovecraft's classic horror story, though it leaves out the article "the" that leads off the story title. As you might expect, the game is based on Lovecraft's Cthulhu Mythos and related tales of cosmic horror, and is considered the largest and most influential horror RPG.
The game was originally written by Sandy Petersen, who later went on to design some of the levels and monsters in the computer game "Doom." Petersen had started writing a supplement for "RuneQuest" set in Lovecraft's Dreamlands but ended up creating something much larger, incorporating more of the Cthulhu Mythos. Since its release in 1981, it has gone through six editions, sold almost a quarter-million copies of the main rulebook, and won over 40 gaming awards.
The primary setting for the game is the 1920s -- New England and especially certain fictional settings from Lovecraft's stories, like Arkham, Dunwich, and Innsmouth, are often considered the default locations. The other most prominent settings authorized by Chaosium include "Cthulhu by Gaslight" (England in the 1890s) and "Cthulhu Today" (modern-day setting). More recent official settings are "Cthulhu: Dark Ages" (the Middle Ages, natch), "Cthulhu Invictus" (Ancient Rome), and "Cthulhu Rising" (23rd century science fiction).
Player characters are generally normal people, with few unusual abilities, who are called Investigators, because their role is to investigate the various strange happenings in the game. There are no character classes or character levels -- it's a percentile-based system focused on skills, so characters can improve their stats by working at them in-game.
The rulebook also included character stats for well-known characters from Lovecraft's fiction, descriptions of Mythos spells and magic items, and a lengthy collection of horrific monsters and deities. But the real breakout feature for "Call of Cthulhu" was Sanity Points, abbreviated as SAN. The more Sanity Points you have, the more mentally stable you are. Unfortunately, there are a lot of things out there that can reduce your Sanity -- even getting a good scare can knock it down a little, but you recover from the small drops easy. The big scares, like the first time you see a nightgaunt or a shoggoth or a deep one, can knock off a tenth of your points easy, and that's going to be enough to drive you insane, at least temporarily. And God help you (which he won't) if you come face-to-tentacle with Yog-Sothoth, Nyarlathotep, or Cthulhu himself -- they can drain your Sanity to zero instantly, and if that happens, you'll have to roll up a new character, 'cause you don't get to come back from zero SAN.
Maybe worse than that -- in order to effectively fight the eldritch forces of the Mythos, you've got to learn more about them, usually by studying horrible blasphemous books like the Revelations of Glaaki, Unaussprechlichen Kulten, or the Necronomicon. And the more you learn, by increasing your "Mythos Lore" skill, the lower your Sanity slips. So the more you learn about them, the more likely you are to go mad.
And even if you don't go mad, you're still playing a 1920s flapper, private eye, or professor, armed with, at best, tommyguns and explosives, battling monsters that are not likely to be harmed by bullets. Characters who don't go nuts are probably going to end up inside a gug's digestive tract.
Chaosium seems to understand that even experienced gamemasters may have difficulty crafting good cosmic horror adventures, so they release many adventure supplements, full of everything from single-night ventures into the unknown to full-blown, multi-session, globetrotting campaigns. The best known include classics like "The Masks of Nyarlathotep," "Horror on the Orient Express," and the "Lovecraft Country" series of supplements, as well as non-Chaosium books, like Pagan Publishing's "Delta Green."
Besides the high quality of the game material, Chaosium also produces props for their games. These are usually simple paper props that are printed in the back pages of their supplements for gamemasters to scan or photocopy to give to players. These range from newspaper clippings to 1920s-era passports to photographs to torn pages from unholy spellbooks. More dedicated fans and artisans have been known to create even more ornate props, including blasphemous idols, pickled specimens in glass jars, and copies of the Necronomicon bound in (hopefully) faux human skin.
Chaosium has developed, over the years, into one of the go-to stops for all things Lovecraftian. Besides game books, they've produced Cthulhu-themed T-shirts, jewelry, fiction and nonfiction anthologies, toys, statuary, board games, and even campaign material for the "Cthulhu for President" promotions ("Why vote for the Lesser Evil?"). Their sole misstep here has been the "Mythos" collectible card game in the mid-1990s, which almost bankrupted the company.
After a few years of severely slowed-down game production, however, Chaosium finally crawled back to the surface. Much of their recent output has been in "monographs" -- shorter books, produced independently by fans and semi-pro writers, and published inexpensively by Chaosium. Not quite the Golden Age of yore, but "Call of Cthulhu" is still the top dog of horror gaming.
Research:
Chaosium
Wikipedia
Game books galore
horrorquest