I get facinated by Mashups a lot. Even more so, whenever it involves videogame music for some reason. The Odd time I stumble on a bunch and it's either good or bad... Usually The Mashups can be pretty bad, but the Odd time there's a real gem in there.

Here's 5 songs that I think are some pretty good mashups: 
Beastie Boys/FF6 - Body Movin'  
Another Bad Creation/CastleVania - Playground 
Snoop Dog/Kirby - Drop it like it's hot 
Jay-Z/Zelda - No Hook 
Jay-Z/Mario - 99Problems