Acadia National Park  has two campgrounds with a range of tent sites and lean-tos. Right on Park Loop Road, Blackwoods Campground (Rte. 3, 800/365-2267, www.nps.gov/acad , $20 per site May–Oct.; free Nov.–Apr.) requires reservations far in advance to secure a spot.
In the southwest corner of the park, Seawall Campground (Rte. 102A, 207/244-3600, www.nps.gov/acad , $20 per drive-in site, $14 per walk-in tent site) is first-come, first-served, but plan on getting there early in the morning during busy summer months to have even a chance of snagging a spot.