Disclaimer: Despite anything I may beleive, Cthulhu and his fellows are entirely a work of fiction.

        In the beginning...
        There was Howard Phillips Lovecraft.  In all honesty, he appears to have been entirely off his rocker, but he could write like no one else.  His collected writings represented a unique (and deeply frightening) mindset.  His roster of shapeless horrors and elder gods is now called the Cthulhu mythos, named after one of their number.  Cthulhu, supposedly, lies beneath the waves, trapped in his city R'lyeh until the "stars are right."
        He has some background and techniques in common with his contemporary, Poe, but conveys an entirely different feeling in his writing.
 
 

        Lovecrafts writings have spawned many things, but only after his death.  His influence can be very clearly seen in authors like August Dereleth and Stephen King (especially his early works).

        Roleplaying
        Lovecraft's writings spawned one of the most resilient role playing games, Call of Cthulhu.  The producers of this game,  Chaosium, still stay true to the form and spirit of his writings.  Settings range from the 1800s to today.  I would highly suggest both Call of Cthulhu and Delta Green, its modern counterpart.  Perhaps moreso Delta Green, but that's just a result of reading the Illuminatus! Trilogy once too many times.

        Necronomicon
        H.P. Lovecraft appearantly invented the Necronomicon as we know it.  The only thing that looks even close is the Egyptian Book of the Dead, which doesn't have nearly the amount of foulness Lovecraft attributes to the Necronomicon.  Not too long ago, several different groups of euntrapenuers set out to bilk the foolhardy by labelling books "The Necronomicon" and selling them in book stores all over the world.
        Lovecraft goes into great detail explaining how the Necromicon drives the reader over the brink of insanity, turning them into gibbering madmen.  This is not the sort of thing that can be mass marketed.  That's why H.P.'s heroes always find it, and tomes like it, in musty old libraries in the dark recesses of Miskatonic University.  If Barnes and Noble had a book in stock that could rip the fabric of time and space, summoning elder gods, someone would probably notice.
        However, at least one of these books has a great deal of actual mythology in it, with just enough Cthulhu and Nyartholtep refrences to keep the profits up.  One of them explains at some length the stories of Marduk and Tiamat, two figures from actual mythology, not a diseased mind.

        Farse

        Cthulhu has created something of a counter culture, with side effects like the Cthulhu fish (above), the Cthulhu Scouts (also above), and the Campus Crusade for Cthulhu.  Most of these people can be respected for the simple fact that they're poking fun at everyone who's gotten too uptight.  What does it matter how the world was created, compared to how we lead our lives.  Evolution doesn't deny God, God doesn't deny Evolution.  Get over it.  Cthulhu does deny God, at least the new testament one, but I won't get into that.

            The Future
            There will soon be some articles added to this page regarding why Lovecraft's writing is so effective, a rundown of the entities involved in the Mythos, and some additions to the Role Playing section of this page on Mythos topics.