Even if you've never read any Lovecraft, chances are you've seen a horror film that used Lovecraftian (yes, really) ideas and imagery. Director Stuart Gordon is particularly Lovecraft-savvy, and two of his most entertaining films are based on Lovecraft stories: Re-Animator and From Beyond. And John Carpenter's underrated In the Mouth of Madness hinges on an idea used in nearly every Lovecraft story -- that there are other worlds in other dimensions, separated from ours by an all-too-thin membrane.
The Lovecraft Archive has a well-organized list of films and TV shows (plus other media) with Lovecraft connections. It's also a good reference on the writer in general.
You can read every Lovecraft story at Dagonbytes' Lovecraft Library. And because it's not a mention of Lovecraft without it: Cthulhu.
Happy birthday, Howard Phillips Lovecraft.