Metaweb’s Freebase has one as well with a data model intended to support
US as well as World postal codes. The base 5-digit US zipcodes are
geolocated, while their extensions are typically not. There are just
over 32,000 zipcodes stored in Freebase for the US.

Access is via either an API, a web client (www.freebase.com) or a
web-based query editor (www.freebase.com/tools/queryeditor/).

All data on the site is free to download or modify.

Examples:

The API query for 90210:
http://www.freebase.com/api/service/mqlread?query={%22query%22:[{%
22/location/location/geolocation%22:{%22latitude%22:null,%22longitude%
22:null},%22country%22:%22United%20States%22,%22name%22:%2290210%22,%
22type%22:%22/location/postal_code%22}]}

The Freebase URL for 90210:
http://www.freebase.com/view/guid/9202a8c04000641f8000000003e57a50

A JSON query in the Freebase query editor for the first 100 us zipcodes:
[
{
“/location/location/geolocation” : {
“latitude” : null,
“longitude” : null
},
“country” : “United States”,
“name” : null,
“type” : “/location/postal_code”
}
]

The same JSON query submitted via the Freebase API:
http://www.freebase.com/api/service/mqlread?query={%22query%22:[{%
22/location/location/geolocation%22:{%22latitude%22:null,%22longitude%
22:null},%22country%22:%22United%20States%22,%22name%22:null,%22type%22:
%22/location/postal_code%22}]}

I’ll be presenting at Where 2.0 on use of Freebase in mashups as a means
of increasing relevance of results and encouraging return visits — see
you there!

Jonathan