Climate Zone tells about -- yes -- climate all around the world. Excellent source of information for regions, countries and states.

Historical Atlas of the 20th Century describes itself as "always under construction."  Great source with clickable maps in deepening layers of detail. Appears to be very accurate, current.

Graphic Maps.com offers free clip art of maps and flags as well as tons of facts about places in the world.

WorldAtlas.com has maps and much more. If you want to find out about world geography and get printable maps, this is the place.

Cartographic Maps is a wonderful collection of JPEG images of maps classified as Ancient, Late Medieval, and Renaissance.

Exploring the West Maps comes from an exhibit at the University of Virginia in 1995. Lots of Lewis and Clark.

National Geographic seems like a logical addition to this list. Interesting basic information, special features, maps, more.

Color Landform Atlas of the United States is a collection of printable state maps in several styles, including ones from a US map made in 1895. Great for
     reports, websites, etc.

Countries of the World tells you lots and lots of things about (you guessed it) the countries of the world.