A Mashup in the world of music, is a mix of two or more songs into one song.
best of bootie 2009
mashup your bootz
these links will provide you with full albums of mashup songs.
you may enjoy them, or you may hate them, one thing is for certain, they will prove very unique.
I will try to continue to update this list with large collections of mashups as I find them. if you enjoy mashups, please post links!

This.
As well as this (I saw this a few days ago, it's bloody shitty).

Whoa, this makes two threads I can contribute to in the same week! I'm on a roll!
25 or 6 to 4 + When I'm 64

Technically it's not really a mashup but it's too awesome.

If you can, lookup for DJ Zebra's mash-ups. He is a french radio DJ (but sometimes does live events too) and frankly, his mash-ups are among the best I've ever heard!!