To add to some of the recommendations (but not all of them...Pearl Jam? Are you kidding me? :p ):
Gram Parsons (really more for the country-rock he perfected with the Byrds and the Flying Burrito Brothers, his solo stuff is a bit too hardcore for me)
Belle and Sebastian (fantastic Scottish pop band that has grown from purely acoustic wistfulness to ballsy rock on their last two LPs. Check out both versions for greater appreciation)
Death Cab for Cutie (based solely on their new "major-label" debut, which is currently in residence in my car stereo)
Old 97s (I only have one album of theirs, "Satellite Rides" from '01. But it's absolutely one of my favorite country-rock albums)
Otis Redding (not just the guy who Duckie mimed to in "Pretty in Pink". Definitely one of the great gritty soul singers)
The Modern Lovers (if you want to see where punk actually began, pick up their one proper studio album in the original incarnation)
The Byrds (not just for the Parsons stuff, but some classic Sixties American pop)
And if you don't have anything by Bob Dylan at this point...what is wrong with you? 