I would add Nick Catanese to the list.... he plays actaul rhythm guitar for Zakk in Black Label... he's also an all around great guitarist... He plays right along with Zakk note for note a lot of times...

Dave Wiener for Steve Vai...

I second James Hettfield...

Izzy Stradlin of GNR...

Dave Grohl of the Foo Fighters...

EDIT!!!

Add John Mayer to that list too... That guy can build a groove like not many can...

And Also Dave Matthews... His playing is so different from the normal acoustic playing... it's so sloppy but produces such a cool rhythm for the band.