First of all, I consider David Allen Coe as part of the "outlaw country" music movement, or at this point in time, a classic country singer and songwriter. I feel as though the outlaw country people should have their own webpage, and that the music is significantly different from what I'd consider folk music.
Robert Earl Keen could possibly be thought of as a folk music guy. He certainly did some folk music on some of his best known albums. He has a great mix of styles, and is not unlike Steve Earle in that.
Of course Mickey Newbury deserves recognition, and was one of the great hippie cowboy guys and a fantastic songwriter. Perhaps another webpage with a different title would be the best place to discuss him. I'm certainly not done writing about the heroes of Texas music.