Only one picture is needed here guys...
Kidding. Although I do have a soft spot for Limp Bizkit (to this day, I still listen to "Counterfeit" and many other of their songs off of 3DBY when I'm pissed), Fred Durst is the posterboy for faggotry.
I think Rage Against the Machine perfected metal/hiphop. I guess that wouldn't really count as a collab since that is what makes RATM, RATM....
Oh well.
Outside of RATM, Limp Bizkit, Kottonmouth Kings, and Slipknot (they have some elements of hiphop.. Turntables, rhythmic vocals), I haven't ventured into rap-metal... Specifically collabs.