ATLAPEDIA - http://www.atlapedia.com/
Atlapedia is essentially an online, very easily accessible atlas. It contains political maps, full color physical maps and key facts as well as statistics about countries around the world. This could be easily used in a Social Studies class for middle school or highschool for the more detailed information as well as elementary with the ability to show students the colour coded countries.
There are three resources; Countries A-Z, World Map, and Class resources. All of which provide a variety of information like climate, people, language, history, economy, homework resources and important facts and figures.
The homework aspect could be very useful in class because students could potentially take work home with them since the site is accessible from anywhere with internet.
There does not seem to be any use restrictions.
