Map Machine by National Geographic

Oldie by goodie - upgraded ~

"MapMachine is a dynamic atlas that lets you explore hundreds of maps -- political and street maps, historic maps from the Library of Congress, population maps, USGS topographic maps, and satellite images."

New features include Aerial imagery, Customization (add-your-own-layers), extensive searching capabilities, and Maps in the News.

Suggested by Jonathan Dube, Poynter Online Web Tips.