Calculate the great-circle distance between two latitude/longitude points online.
Compute the great-circle (haversine) distance between two points given their latitude and longitude, in kilometres, miles, metres and nautical miles.
The shortest distance over the Earth's surface between two points — what you want for "as the crow flies", not road distance.
Haversine assumes a spherical Earth, accurate to ~0.5%. Fine for almost all distance needs.