Every year for the last three years, the Maine Historical Society (mainehistory.org) has held a one-day event on a weekend in May when they give lucky ticket-holders a chance to tour through areas of approximately ten different historic sites that are normally off-limits to the general public.
I've managed to go every year since they started, come hell or high water, and it's been great. Here are a bunch of photos from this year's tour on my Flickr page! I think I managed to get almost all of them labelled, but if you have any questions, please feel free to ask!
Here are a few samples: