This service uses data from the US Census and a supplementary list of cities around the world to find the latitude and longitude of two places, and then calculates the distance between them (as the crow flies). It also provides a map showing the two places, using the Xerox PARC Map Server.

Various query formats are allowed; for example:

Chicago, IL
Jakarta
40:26:26N 79:59:46W
Athens, Greece

(Info on the image above.)