I really like this kind of stuff. Some of my favorite things to whip out on unsuspecting audiences are a slow country version of "you shook me all night long" (with a lot of b-bending), and "Pinball wizard" done to the tune of "Folsom Prison Blues".
One of my favorite performance mash-ups that I often play with my sister is Journey's "Any way you want it", turning it into Tina Turner's "I will always love you", and then turning that into the Proclaimers "500 miles".
It is so fun to play and always gets great reactions.
One of the oldest that I have heard, before Party Ben's Mash up of Green Day and Oasis, and before the Grey Album, was Little Roger and the Goosebumps, "Stairway to Gilligan's Island." just try the those two on top of each other, or look up the song... Wait, of course it is there...
you can listen to it there. Somewhere between awful and genius.