AKUS, Doyle Lawson, Dailey and Vincent, O Brother soundtrack, Del McCoury, Mountain Heart, DBFS, O Mickey Where Art Thou? (mostly for the kids but Randy Kohrs is AMAZING), Seldom Scene, Blue Highway, IIIrd Tyme Out, etc.
Pandora is a great app. It's ad supported but at least the ads are unobtrusive. Slacker radio ads have audio and are really annoying.
I wish there was more support for adding lyrics to bluegrass songs. I use iTuneFriend and that caught a few of my bluegrass songs, but not most.