Ya, Johnny's cover of, "Hurt," is sweet. Pandora radio has helped me realize Johnny Cash has made some really awesome cover songs. I had no idea he had such an emotional style of singer/songwriter of his own, and it's awesome.
The cover of, "One," is another great example, originally by U2.http://www.youtube.com/watch?v=DQIrxhNkiAs
My favorite cover probably is The Grateful Dead's, "Scarlet Begonias," done by Sublime.http://www.youtube.com/watch?v=4Cg2O4SsHQw