The Hood Internet

The Hood Internet might be the best mash up creators I know of. Every mashup I've featured of theirs (check out the other features by going to the somewhat newly added 'tags' in the right column) is so clean and perfect. They're on top of their shit, remixing new music all the time. Here's a couple of the hip hop class of '09 mashed up, directly from their website (linked above).

ABX - I Love Friction (Asher Roth vs. The Pains of Being Pure at Heart) (direct)
The 'chug, chug, chug, chug, etc.' part is dirty.

DJ STV SLV - Another Day 'N' Nite (Kid Cudi vs Cornelius) (direct)