Travelmath provides an online flight distance calculator for all types of travel routes. You can enter airports, cities, states, countries, or zip codes to find the distance between any two points, measured by the great circle distance formula. The calculation is done using the Vincenty algorithm and the WGS84 ellipsoid model of the Earth, which is the same one used by most GPS receivers. This gives you the flying distance "as the crow flies." Find your travel distance quickly to estimate the number of frequent flyer miles you'll accumulate. Or ask how far is it between cities to solve your homework problems.
Maybe you want to see the flight time instead? Or if you're thinking about driving, compare the driving distance for the same trip. You can also find the nearest airport to a city, or explore major airports in any state or country.