Adding to the list, mostly Texas artists but Guy Clark (leaving us soon), Robert Earl Keen, Slaid Cleaves, James McMurtry, Hayes Carll. These are true storytellers in the vein of Merle and worth looking into.
1. https://www.youtube.com/watch?v=fYtiE3cAEC0
2. https://www.youtube.com/watch?v=gBcM96iopPo
Edit: How could I forget Jamey Johnson? Here's one of him alongside Robert Earl Keen and Raul Malo: https://www.youtube.com/watch?v=dajb94tV-eM
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).