A friend recently introduced me to a great IPhone App for the website Daytrotter, which hosts exclusive versions and unreleased tracks for an impressive list of artists.  The interface on the site isn't quite as handy as the IPhone App but that is more than made up for by giving you the option to download all content in MP3.  The number of sessions is a little overwhelming (currently 772!) so here's a few to get you started. 

Black Joe Lewis & the Honeybears
Backyard Tire Fire
Blitzen Trapper
Brendan Benson
Cage the Elephant
Grace Potter & the Nocturnals
Mason Jennings