Is there anything Wikipedia doesn't cover?  I think not.  I assume you've seen one or two of the movie trailer mashups that have made the rounds, such as Shining, Brokeback to the Future, or the latest George Lucas Presents: Singin' In The Rain.  Wikipedia has a page that links to more trailer mashups than you can shake a stick at!  (Well, you probably could shake a stick at them, actually.  That's a dumb expression.)

And here's one for a little instant gratification.

American Pie as a horror flick