Second that. Also, forgot to mention Non Phixion in my first post. Masta Ace is also good, and I'm partial to Apani as well, though her racks seem to be the most difficult to download, or at least I can't find them.
Like iamthewolf (jedi mind reference or does that get mentioned elsewhere as well?), I could go on and on about this topic. While I'm at it, I think Fort Minor and the Jay-Z / Linkin Park crossovers deserve a mention. I'm especially partial to Remember the Name and Lying from you / Dirt off my shoulders.