UPDATE: I neglected to give you a bunch of links where you can download the mashups here and others I found. Those links are now included so you can give the artists, DJs and the geeks who support them some love.
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 bastard popmusical 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.” 
In full swing at the start of the 21st century, mash ups have been described positively are “ultimate post-modern pop song[s]” or “‘culture jamming in its purest form'” and “display an analytical impulse…we actually hear how these songs resonate with one another, comment upon and affect one another…in a way, the music speaks for itself” . They have also been described negatively as “the logical extension of the sampling fever of the ’80s taken to its dumbest extreme”. Due to the questionable legal status of mash ups and little chance of profits “there’s a real punk rock attitude attached to the movement” .
So there you have it 🙂
Battle Funktastic – Hotei vs. Rip Slyme (you’ll have to search for it, because it took me forever to find it after hearing it in 2006 when I was in Tokyo, which is the only place I’ve ever heard it other than my iPod)