History Maps is a simple, yet effective, app - that puts a large collection of high resolution historical maps into the palm of your hands. You can choose from a wide selection by category or by era, ranging back as far as the 4th century.
Although you can probably access all of these maps online, it's great to have them in one central location and available offline. The high resolution allows you to zoom in to examine the detail on each one. It's also available on iPhone, iPod, iTouch.
Oh, and it's free!