So I guess there isn't a Music Category (shouldn't there be?) I will place this in art.
Some Mashups suck such totally balls. However others are mind blowing.
I suggest you check out Girl Talk. http://en.wikipedia.org/wiki/Girl_Talk_(musician)
Here is some of his awesome stuff
The videos end rather abruptly. But that is because they are meant to be listened to as an album and the the songs run into each other.