I;m surprised no one has mentioned Newcastleton, first weekend in July. Although there is a smallish charge for some of the events there is no ticket charge per se. Camping on the main site is chargeable but you can camp by the river (5 minutes further away) for free.

Several sessions, both tune and singaround, in the pubs, the club, the village hall and the camp sites.

www.newcastleton.com