The best beach in Maine, in my opinion, is in Moody, ME - just between Wells and Ogunquit, and is referred to as The Footbridge:
http://ogunquittownhouse.com/Footbridge.html
http://www.visitmaine.net/beaches.htm
We like the Footbridge because it's clean, easy to get to, parking isn't terribly expensive and there's public parking, public restrooms are pretty clean, there's a snack shop that's not outrageous (or all that special - but it works if you need chips and a drink) - and the beach isn't rocky.
It's a unique beach - and you get to it by walking about a quarter of a mile over a footbridge. The beach is very (very!) long - you won't run out of space even during high tide.
It's our favorite place to bring the kiddos. (But not our favorite beach to bring the dog - this beach has a NO DOGS policy all year 'round.)