Geozip
Geozip is a Perl program for calculating the distance between two zip codes in the United States. The sub-routines included in the code can be used to calculate any distance between latitude and longitude coordinates. The package includes data for the US, the script, including two sub-routines, one gets the coordinates and city name, the other calculates the distance between two points. The result should have a precision of about 50 meters, because the calculation takes into account the fact that the earth is not a perfect sphere. A great program to integrate into e-business solutions, dating sites, or just as it is as a service to your visitors. Update: Now with PHP/MySQL routines!