The 13 writing promts by Dan Wiencek (posted at McSweeney's) are the most original I've seen yet. Here are the first three...
Write a scene showing a man and a woman arguing over the man's friendship with a former girlfriend. Do not mention the girlfriend, the man, the woman, or the argument.
Write a short scene set at a lake, with trees and shit. Throw some birds in there, too.
Choose your favorite historical figure and imagine if he/she had been led to greatness by the promptings of an invisible imp living behind his or her right ear. Write a story from the point of view of this creature. Where did it come from? What are its goals? Use research to make your story as accurate as possible.