Ok, so this isn't exactly a mash-up, but it's a piece that brings together two things that have no business sharing the same screen-space, and makes them fit incredibly well. Everyone, meet 8 1/2 Mile, a trailer for Federico Fellini's masterpiece 8 1/2, set to the soundtrack of Eminem's Lose Yourself. I'm totally going to lose all my credibility as a snobby lover of foreign films here, but I have to admit that it gave me chills. Sure, it's not the best representation of what, exactly, the movie is about, but what trailer is? It creates a wildly effective atmosphere, and the editing is perfect. Plus, you get to look at the wonderful Marcello Mastroianni -- basically, there are a lot of worse ways to spend two-and-a-half minutes on a Saturday. (Of course, if the end result is that you, like me, find yourself seized by a desperate need to go watch 8 1/2, the time commitment is going to be a little longer.)

Edit: The original link stopped working; links now point to a reliable site.