The A1 Diner, Gardiner, Maine.
In order to get to Gardiner, we went through Augusta...and actually passed by a few other intriguing-looking spots, including a funky-looking drive-in burger joint, reminiscent of the old A&Ws I used to go to as a kid. May be worth a return visit.
If you're looking for the quintessential New England dinah, this is the place. It's not too far from the interstate, either, which is a definite bonus if you're passing through. Not touristy in the least, although you can pick up a bit of swag...postcards, the Guy Fieri book, etc., either at the counter or the website.
I had the steak and eggs, and couldn't believe how awesome the steak was...we asked, and it's locally-grown...didn't think it could make that huge a difference. Very friendly service, and the back and forth between the youngish fella behind the counter, and the older neighbourhood lady with the thick Maine accent made the experience complete.