Groundhog (Marmota monax), aka Woodchuck, Whistle-pig, Land-beaver, 5/2/12 PL, DE, inset is juv. 5/22/12 PL DE

This young one has a broad white stripe across its nose.
Size: Roughly speaking - adult=cat/young=squirrel.
http://en.wikipedia.org/wiki/Groundhog
The Groundhog/Woodchuck is a rodent in the same family (Sciuridae) with large ground squirrels.