US zipcode/city/county/state mapping
These are "ZCTA"s, which are based on USPS zip codes but more directly relate to spacial information (I.E. you can get lat/lon boundaries for them). From the article:
ZIP codes are a very messy kind of geography. They were created by the U.S. Postal Service as a tool to help deliver the mail more efficiently. ("ZIP" is actually an acronym for "Zone Improvement Plan", where "Zone" is a reference to the 2-digit postal zones that were used by the post office prior to implementing nationwide ZIP codes back in the early 1960's. Because it is an acronym we always use the uppercase for it.) ZIP codes have been adopted by marketing people and by all kinds of other researchers as a standard geographic area, like a city or a county. We see maps of ZIP codes in telephone books and from commercial vendors that make us think of them as spatially defined areas with precise boundaries, similar to counties. But, from the perspective of the agency that defines them, the U.S. Postal Service, ZIP codes are not and never have been such spatial entities. They are simply categories for grouping mailing addresses. As such, ZIP codes do in most cases resemble spatial areas since they are comprised of spatially clustered street ranges. But not always. In rural areas, ZIP codes can be collections of lines (rural delivery routes) that in reality do no look much like a closed spatial area. In areas where there is no mail delivery (deserts, mountains, lakes, much of Nevada and Utah) ZIP codes are not really defined. You may see maps that show ZIP code boundaries that include such areas, but these are not post-office-defined official definitions. An area will not be assigned a ZIP code until there is a reason for it, i.e. until there needs to be mail delivered there. So the actual definition of a ZIP code "boundary" is quite fuzzy at best, and a purely extrapolated guess (at what it would be if someone were to start receiving mail there) at worst. If you have an application that requires extreme geographic precision, especially in sparsely populated areas, then you need to avoid using ZIP codes.
Data is available in .csv and .yaml format.
Example usage of the yaml data:
require 'yaml' zipdata = YAML::load( File.open( 'geo-data.yaml' ) ) puts zipdata["18337"].inspect