This collection of historic maps is truly impressive. It boasts more than 20,000 images available online (out of a total collection of more than 150,000). Although most of the online images are of maps from 18th and 19th century America, there are also several world maps included.
You need a viewer to see the collection and several are available on the site. Probably the easiest to use is the Luna 6.0 Browser: when you click on the browser, it activates with no download...just use it for you session and then it's gone. It will work with IE, FireFox and Safari. Available also is a Java Client that requires a one-time download if you prefer. All of the options are available on the home page and clearly explained.
Here is the easiest way to view the collection: from the home page click on "View the Collection" located on the left of the screen, then click on the Luna Browser option. The next window that appears will give you viewing options on the left of the screen. You can browse the collection by the general categories of where, what, when, or who. And at the bottom of the list is an advanced search option that lets you be as specific as you wish.
Check out this collection at: http://davidrumsey.com/view.html