Lovecraft's fiction dealt with mysterious, all-powerful beings that are somehow sealed away from our existence, yet constantly try to break in (with the help of savage cults, quislings and power-hungry men); his most famous creation is Cthulhu, the high priest of the Elder Gods, who lies in endless sleep in the sunken city of R'lyeh waiting for a time 'when the stars are right' so that he might awaken and wreak havoc over the globe. In HPL's 'The Call of Cthulhu', he is momentarily awakened, but since the stars aren't right he is again trapped in his watery sepulcher.
Cthulhu, by the way, is a monstrous winged being with tentacles all over his face.
Other Lovecraftian creations are Nyarlathotep (the Masked Messenger, the Crawling Chaos), Azathoth (the idiot god at the center of the universe), Yog-Sothoth, the Mi-Go (intelligent fungi from Pluto), Shub-Niggurath (the Black Goat of the Woods with Ten Thousand Young) and so many others...
'Lovecraftian' fiction shares some common elements: there is usually at least one ancient tome mentioned (Lovecraft's most famous is the Necronomicon, by the mad Arab Abdul Alhazred), there are usually tentacles/slimy monsters/vaguely vaginal creatures, there's usually some effort made to bring the Elder Gods back into our world, the heroes tend to be scholars and academics rather than action-types (though this is changing recently), and there's usually a gloomy ending - the gods may not have broken through, but someone dies or goes insane.
Hope that helps a little...
Oh, and I'm all for a 'versus' anthology; sounds wicked cool!
Or werewolves. I love werewolves.