What is a mashup? Wikipedia:

"A mashup or bootleg^[1] is a song or composition created from the combination of the music from one song with the a cappella from another (also mash up and mash-up). These songs may be considered part of the European bastard pop musical genre. "A mash-up is a song created out of pieces of two or more songs, usually by overlaying the vocal track of one song seamlessly over the music track of another... Mash-ups are incredible fun and a fascinating way to reexperience some of your favorite tunes.""

And there is this website - offering over 200 tracks and extras around the mashup theme - even best-of-samplers are available:

Source 1: http://en.wikipedia.org/wiki/Mashup_(music)
Source 2: http://www.bootieusa.com/