Census Maps - http://factfinder.census.gov/java_prod/dads.ui.homePage.HomePage
The U.S. Census Bureau's mapping service that provides quick thematic maps (population, income, housing statistics, etc.) and detailed customized maps.
Globalis - http://globalis.gvu.unu.edu/
Interactive, printable world maps on a variety of themes and statistics; produced by the United Nations Environment Programme and the Global Virtual University.
TerraFly - http://terrafly.fiu.edu/
Enter an address and take control of an interactive satellite imaging system. Brought to you by the High Performance Database Research Center at the Florida International University School of Computer Science.
Terrestrial Ecoregions of the World - http://www.nationalgeographic.com/wildworld
National Geographic combines interactive mapping with ecology. Enter your zip code, a habitat type, or any place name to retrieve maps and basic information for that area.
Your Sky - http://www.fourmilab.ch/yoursky
Your Sky (not related to the physics institute, Fermilab) is created and maintained by programmer John Walker, and allows users to view the entire sky or a horizon from any latitude/longitude, or from a list of cities. Customized sky maps include constellations, asteroids, comets and other celestial objects. Includes a useful "Help" section.