Afrobarometer is the archive for a huge amount of statistical data about modern Africa. Most of this kind of data did not exist until about twenty years ago, and all of it is amazingly valuable. All sorts of political, social and economic data are gathered here.

The NPS' website has a series of links to all manner of useful maps, including ones hosted by the University of California, that are quite good.

The Metropolitan Museum of Art in NY, NY, has scans of a large amount of their collection online. This can be really useful to help students visualize a period of history and make connections between the history they are learning and the people, places, and events involved.

This website is amazing. Type in a zip or postal code (for the U.S. or elsewhere) and the website will help you overlay an event for your students. Its makes it easy to see how much area would be affected if the BP oil spill happened in your zip code, or mow much space Bejing in 1425 really covered. Fascinating.

This collection, run by the University of Texas, contains maps of all sorts from a large period of time. Additionally, it also houses links to collections not run by the university, but nearly all of both groups of maps are excellent scans of the original documents.

This is a highly useful source, a derivative of the Internet History Sourcebook series, which focuses on African history. It is particularly helpful as it's full of difficult to locate sources for periods that are often dependent on oral histories and secondary sources.