October Project. I only know this one song, but my wife and I saw them when they opened for Crash Test Dummies.
http://youtube.com/watch?v=dZSMK_w3f3U
Alison Krauss, from O Brother Where Art Thou"
http://youtube.com/watch?v=F1FQqSGxBso
Ralph Stanley, also from "O Brother Where Art Thou"
http://youtube.com/watch?v=9E8S3tCWmiQ
This is slightly different from the movie version.
Just see the movie and get the whole soundtrack. It is an amazing homage to early twentieth century american music.