yeah yeah i know Girl Talk are the kings of mashups, but i truly love pretty much everything the Hood Internet does. The R Kelly/BSS is totally inspired, and i've recently been listening to this a lot:
http://hypem.com/#/track/1062386/The+Hood+Internet+-+Decalogue+The+Hood+Internet+vs+The+2000s+

This one looks back over the past decage by mashing up two songs from each decade over a sweet 6 minutes. Some of it is just awesome, the Arcade Fire vs Kanye West/Twista is so awesome!

So, as a side note, what are your favourite mashups, and post them here for us please!