More details and the full schedule is at http://www.dominionriverrock.com/schedule/
The Riverock festival is free and includes a bouldering competition (there may be an entrance fee to compete). But the concerts are definitely free.
I've never been, but I always wanted to go. I just booked a room at the Crowne Plaza Downtown, less than a half-mile from the park. If you're looking for someone to share a room with, post a comment.
I've decided to drive down Saturday morning instead of Friday night. If people want to go just for the day on Saturday (or Sunday), that's OK too.