Geobytes, Google Maps and Tripit all offer methods of calculating the distance between two cities. Each user-friendly site provides a different way to measure the distance.
Geobytes allows users to select either air or road distance and calculates the precise distance between the origin and destination. Air distance is "as the crow flies," while road distance uses existing roads and routes.
Google Maps provide road distances between two cities in general, or precise distances from point to point with driving instructions.
Tripit allows users to enter a city, country or airport and calculate the distance between. The site does not specify whether the measurements are by land or by air.