Latitude and Longitude calculations

TV Infomercials, Zip Codes and The Haversine Formula

Distance between locations using latitude and longitude

Determining if point falls within a polygon where point and polygon is defined by lat/long coordinates

Zip Code database

At SourceForge

The Zip Code Database project exists to provide US Zip Codes in
their entirety; latitude and longitude coordinates included! See the
home page for the distance calculation algorithm.