[SL__72]

I don't think he qualifies at all... I don't think he "writes" any of the music for Tool either, other then maybe the lyrics. From the few interviews I've read/heard its more of a collaborative process with them.

Ryan Adams is a great nomination.

I would also nominate Jim James from My Morning Jacket, easily one of my favorites.
Willie Nelson?
Waylon Jennings?
I'm not too sure what % of their music each wrote, but even if it was only 50% I would consider both of them along with the already mentioned Johnny Cash.