I'm sure nearly everyone is familiar with Girl Talk, but he is definitely up there...
(not sure if he fits everyone's definition of "mash up," though)
http://en.wikipedia.org/wiki/Girl_Talk_(musician)
The Aesop rock/sufjan Stevens "none shall pass/star of wonder" mash-up is my all-time favorite.
I like the ones they did on that episode of Glee.