There are a ton of maps and other geographic information available on the web. Because there's so much it's sometimes hard to find the right kind of map for what you are doing. Here are some starting places:
worldatlas.com--A fair collection of physical and political maps.
Infoplease.com United States Atlas
Infoplease.com Atlas--Better for political maps
Mapswire.com--Limited number of maps but pretty good quality.
Wikipedia list of Sovereign States--Not an atlas, but organized in easy to use format. Each country entry usually has a good map or two.
The Map as History Some are available, others are behind a paywall
Geachron--Maps of the world over time
Geoguesser A geographic guessing game. Try to figure out where you are.
Map Crunch--Sends you to random Google Street view places
Global Carbon Footprint by country