Geo / ZipData / 0.2.0
Takes a zip code, returns various pieces of relevant data for that zipcode. If not found, returns null.

The data includes zip code, zip code type (standard or PO box), city (beware that some zip codes span multiple cities), state, latitude, longitude, whether or not the code is decommissioned, and estimated population.

Based on, using data last updated 1/22/2012.