Well recently someone posted an awesome mashup in the beatles thread, and since then i've found a few others.
The Beatles vs Joan Jett vs Cypress Hill vs House of Pain vs RATM - Mash Together (from beatles thread)
[media]http://www.youtube.com/watch?v=MZBuYbKgvQI[/media]
Jayz vs. Hendrix 99 Problems / Voodoo Chile
[media]http://www.youtube.com/watch?v=cqpXmwSUmGg[/media]
Ray No Speak Americano
[media]http://www.youtube.com/watch?v=0bFymamXBEo&feature=related[/media]
The Beatles vs LCD Soundsystem vs The Kinks
[media]http://www.youtube.com/watch?v=MPtWh5XjiH0&feature=related[/media]
So post more mash ups you have found, the best are ones that span several genres.
[MEDIA]http://www.youtube.com/watch?v=arLbC8VxB_M[/MEDIA]
This could be my favourite, but it isn't greatly made
Take Five meets 15 step
[media]http://www.youtube.com/watch?v=TYa7furgQsA[/media]
Wish Take five was more prominent than 15 step but oh well.
[url=http://djlobsterdust.com/index.php/mashups/deep-purple-vs-daft-punk-vs-usher-vs-ludacris/]Lobsterdust - Funky Smoke (Deep Purple vs. Daft Punk vs. Usher)[/url]