The SteelDrivers. If you have not ever listened to the SteelDrivers you are seriously missing out. They're more of a modern bluegrass group with a ton of talent. Their songwriting and delivery is outstanding. Please do yourself a favor and check them out. "Blue Side of the Mountain" WILL be stuck in your head. (Oh and if you think they're good on CD, you need to see them live because really, they're that much better.)
Dailey and Vincent. I think they need to come up with a new name that includes the band as a whole, but the music makes up for it ten fold. They are a more traditional bluegrass band with 3 and 4 part harmonies, a fiddle, upright bass, guitar, mandolin, and banjo. Although I had a hard time getting past some of the sound idiosyncrasies (you would think for the hundreds of amazing sound engineers in Nashville, they'd at least have a decent one), they were pretty tight and very talented. They have been together less than a year and are up for 10 awards at the IBMAs.
Kathy Mattea. You have probably at least heard her name. She has an incredible voice and knows how to use it properly. Mattea has a new album out that, from what I've heard so far, is fantastic.