These are a couple of years old, but they popped up on my Google alerts today for some reason.
The second one takes a little while to get going, but it works. With both songs I like that the person didn’t just layer the vocals over another track and let’s hope to God they fit (or worse, just fade from one song to another and back again). Apparently they had to isolate out the vocals, too, so kudos to them for putting some work into it.
(And of course if I’m talking about mash-ups, I have to include my fave: