I'm going to preface this with an acknowledement to my brother, because if I didn't, I know I'd get all sorts of grief about it. I learned about the group Old Crow Medicine Show from him. And the link below is to one of the first songs I heard of theirs. Its pretty spiffy. If you liked the music from O Brother, Where Art Thou?, then you'll probably like this stuff.
Wagon Wheel- Old Crow Medicine Show