GeoPlaces Web Service
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GeoPlaces Service is the evolution of ZipCodes web service. It enhances zip code lookup by adding support for cities, towns, villages, etc.
Now you can find near-by zip codes and cities, find distance between zip codes and cities, get coordinates of zip codes and cities.
The service also supports user-defined places. You can upload a list of your locations and perform searches on them similarly to zip codes and cities.
This service requires a subscription. Monthly and yearly subscriptions cost a little over $1 per month. A free option is also available.

Download C# client for GeoPlaces web service.

ZipCodes Web Service
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ZipCodes service contains functionality to search for near-by zip codes within a specified distance.
The service also supports user-defined places. You can upload a list of your locations and perform searches on them similarly to zip codes and cities.
This service requires a subscription. Monthly and yearly subscriptions cost a little over $1 per month. A free option is also available.

Download VB 6 client and VB.NET client for ZipCodes web service.