All available for private use at no cost!
The collection is amazing in it's depth and breadth. I've already found maps my local museum has that I can actually take screen shots of to have in hand. Many of the old original maps are literally untouchable let alone being able to copy them. I've had to have the curator in the room to look at some of the 35th Parallel maps I like to research.
http://www.davidrumsey.com/Welcome to the David Rumsey Map Collection Database and Blog. The Map Database has many viewers and the Blog has numerous categories.
The historical map collection has over 21,000 maps and images online. The collection focuses on rare 18th and 19th century North American and South American maps and other cartographic materials. Historic maps of the World, Europe, Asia, and Africa are also represented.