Remix
Everybody's talking about this, so you probably know about it already, but:
Have you seen The Shining? If you haven't, this won't be as entertaining, but all you really need to know is what you probably already know: it's a really creepy classic horror movie starring Jack Nicholson and Shelley Duvall, directed by Stanley Kubrick, based on a Steven King novel. Here's the (really creepy and disturbing) original trailer (which you definitely don't need to see to enjoy the following).
And here's the remixed trailer put together by Robert Ryang, a film editor's assistant. It recently won a contest run by an editors' association in NYC; it's a remarkable example of what editing can do. For more information, including info on what the contest was about, see the New York Times article.
Since then, a couple more people at the company where Ryang works have released remixed trailers for other movies: one and two. (I think those ones may be a little more effective if you don't know from the start what movies the footage came from.) Neither of those is nearly as impressive as the Shining one, but still interesting.