My favorite summer festival is GratefulFest! I've been to, I think, 4 of the last 5. or something like that. It got a little big last year and I was a little worried about crowd size for the infrastructure and all, but it worked out. I'm working my ass off now to be able to take the time off of my (day) job to get back out there this summer.
A few years ago now, I was at more loose ends for the summer and got to see GratefulFest, Great Blue Heron and BlueGrass. I had tickets for Grassroots, but an unfortunate event with a State Trooper kept me home for that weekend.