I recently came across a great website for browsing or viewing old maps, the aptly named Old Maps Online.
Old Maps Online is a collaboration between The Great Britain Historical GIS Project based at The University of Portsmouth, UK and Klokan Technologies GmbH, Switzerland, and despite its European origins, has an incredible wealth of old maps worldwide.
When you first navigate to the site it will automatically take you to your detected location. Many of the maps are available in very high resolution.
At the moment I'm particularly enjoying this map of the Connecticut Western Reserve from 1833.