What is it about Orcs?

What is it about orcs that seems to have captured the imaginations of fantasy fans? I don't see books with elves or halflings, but orcs have migrated into the fantasy mainstream in droves, from Mary Gentle's Grunts to Stan Nicholls' Bodyguard of Lightning and its sequels to, more recently, Scott Oden's A Gathering of Ravens. They're in video games, movies, literature . . . what is it about them that creators seem to love?

And, who does orcs closest to Tolkien's own?