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Maps

AirNow - Air Quality reports and forecasts
Acme Mapper - Very nice free satellite image maps
EnviroMapper
Globexplorer - Zoomable satellite Image maps. I think they have a larger selection of newer satellite images. However the map screen is a small and fixed size. Also they are slower to load and their logo interferes with the image. When you do a search it opens an a separate window.
Google Maps - I like google maps satellite images better. The images I saw were a bit older but the map screen adjusts itself to fit your browser window. It loads fast and their logo is readable but does not obstruct my view of the city. And they now have a new feature called Hybrid which is an Overlay of a street map over the picture and it works with the zoom in.
HUD E-Maps - Detailed environmental Maps
Mapblast - MSN Maps and Directions
MapBuilder - Use Google Maps to create maps for web pages
Maps and cartograms of the 2004 US presidential election results
MapTech - Topographical maps. Click Maps Online at top right
MapQuest - They don't have satellite images and their map screen is a small fixed size. But they seem to be fast and fairly accurate.
MSN Virtual Earth
TerraFly - Zoomable satellite Image maps. With local info.
Terraserver-usa.com
UT Library Map Collection - very nice world, political maps...
United States Maps - Perry-Castañeda Library Map Collection
U.S. Gazetteer - Very useful, shows zip codes on maps etc...
World Fact Book - Maps of the World.
Yahoo Maps Beta

World Wind - Free from NASA. lets you zoom from satellite into any place on Earth...

Weights, Tables, Calculators and Measures

Flags of All Countries
Google's Built-In Calculator
How long it will take to become a millionaire
Metric System
Motionnet
Online Conversion
Periodic Table of the Elements - This is a really neat government website presented by Los Alamos National Laboratory. The front page shows a color coded version of the Periodic table complete with all the standard things you would expect including Lanthanides and Actinides. When you mouse over each elements it shows the full name of the element and a picture representing that element. Click on any element to learn about the History, including how it was discovered, where is can be found, what it can be used for and much much more. On the download page there is a printable version both in color and black and white. But there is more. There is even a 716 page PDF file containing the complete Website. If you are interested in high school chemistry, this is the site for you.
Population Clock
Salary Wizard
Temperature Conversion Calculator
Visual Elements - Visual Periodic Table, very nice.
World Flag Database