Turanil said:
			
		
	
	
		
		
			Ditto. Note that CoC d20, while not being printed anymore is still available on the Internet (amazon.com). While CoC d20 only deals with Cthulhu horror, OGL horror deals with all sort of horror genres.
		
		
	 
I'd take exception to that claim, Turanil.  CoC has the best advice I've ever seen on running 
any kind of horror game (disclaimer: I don't own OGL Horror, so I can't comment) and for classic horror monsters; those are mostly in the 
Monster Manual anyway (vampires, zombies, werewolves, Frankenstein monsters, i.e.; flesh golems, etc.).
Rules-wise, I actually prefer the 
Darkness & Dread take on horror checks and Sanity than the CoC one, though -- I think CoC was ill-advised to bolt on the BRP mechanic without even doing any work to make it "native" to d20.  And I think Ken Hood's Grim N Gritty Hit Point system is the better than simply doing the CoC Massive Damage Thresh-hold.
I guess my favorite horror game would feature mechanics cobbled together from two different sources and GM advice from a third.   :\   It'd be nice if it were all in one place.
Are you looking for standard actual horror, or just adding a horror element into a D&D game?  I think the answer might be different (from me, anyway) depending.