And female artists in a similar vein: Lucinda Williams, Brandi Carlile and Gillian Welch immediately come to mind.

A great site (IMHO) for keeping up with musicians in the style of these guys is Jessie Scott's musicfog.com. Jessie Scott was the program director for X Country on XM, before it was replaced with whatever Sirius runs there now and it was her programming that really expanded my music base (I would likely have never heard of any of these artists if not for X Country).