I've always liked the Winfield (Kansas) Bluegrass Festival, though I haven't been able to attend in many an aeon. Features a wide variety of folk music and musicians (I've seen Bryan Bowers, De Dannan, David Grisman, Marc O'Connor, Norman Blake and Hot Rize in the course of a single day, for example). Winfield is also home to national flatpicking and fiddle championships. It's a summer event, 3 or 4 days in duration and well worth the trip to the hot, dry plains of Kansas.