As the Crow Flies: To find the distance between two addresses "as the crow flies," use the GP Visualizer. As a back-up you can use the Gmaps Pedometer with the radio button clicked for "manually (straight lines)," but it's harder to use; you might want to read the the Usage Instructions. To find the distance between two or more cities, go to Bali & Indonesia on the Net's distance: how far is it. Alternatively, OAG flight guides list the mileage between cities with airports. The flight guides are available in many public libraries, or they can be can be purchased in print, CD-ROM or online from OAG.
Distance on the Ground: If you would like to plan out your own route, especially if you are walking, use Google Maps or Gmaps Pedometer (check out the helpful Usage Instructions). You can also use the resources in "Driving Distance" section, below, if you have are traveling between two points with known street addresses.
Driving Distance: To find the shortest way to drive from one place to another in the U.S., use Google Maps, Mapquest, bing Maps and/or Yahoo! Local Maps.
Miles: When we refer to miles, we usually mean 5,280 feet (1,609.344 meters), also known as a "statute mile." There is also a "nautical mile" of 6,076.12 feet (1,852 meters) and a "survey mile" of 5,280.01 feet (1,609.3472 meters). Note: If a contract refers to "air miles" it could mean (1) "as the crow flies" (e.g., in a non-compete clause); (2) the same distance as a nautical mile (e.g., if used in connection with aviation) or (3) a promotional unit offered by an airline or credit card company.
{Thanks to Jonathan Montgomery for the tip on GP Visualizer and to Beth Morey for the tip on OAG flight guides.}