I have a few:
Animals as Leaders - Animals as Leaders
Great driving music
Definative - INXS
I know complination albums are usually out-of-bounds in threads like this, but I couldn't go past this.
I drove from the West coast of Australia to the East, only stopping to change drivers and refuel - this and the above album were on shuffle just about the whole way.
Dark Passion Play - Nightwish
This is my choice if I'm in the mood for something a bit heavier - I love the contrast between the male and female vocals, as well as between the heavy guitar work and the symphonic elements.
The Sufferer and The Witness - Rise Against
This is where I go to mix things up a bit.
The bad show a good range here - from the slow acoustic of 'Roadside' to the ballad-like 'The Approaching Curve' to the thought-provoking (to me, anyway

) 'Survive' and 'Prayer of Refuge'
Yep, if I was stuck with just these four albums for the rest of my time here, I'd be pretty happy.