The sample javascript code uses jquery-jsonp to query the data. During testing this library seemed to have the best error handling and flexibility available. Two key fields are required to make this work as expected. The URL field must include the zip code being queried as part of the URL instead of as a parameter. Additionally the callback must be set to "zipcall". This is due to each request hitting a distinct generated json file. This was done to ensure uptime, scalability and response time. A publically available resource is available at zip.50projects.com. This is backed by Amazon's Cloudfront CDN so you can be assured that presenting it to your own users will be extremely quick. A tar file of all of the backing data is available as well and a short summary of the contents is below. #jquery-jsonp URL http://code.google.com/p/jquery-jsonp/ #Example url: "http://zip.50projects.com/zips/"+request.term+".json", callback: "zipcall", Independent JSON files were generated for each zip code. In some instances there may be two city entries for one zip code. The first entry is the USPS official city name. The subsequent entries are alternate but still valid names. $ cat zips/98133.json zipcall(["SEATTLE, WA 98133","SHORELINE, WA 98133"] Additonally aggregate zip files were generated for auto completion purposes. Each of these is sorted via population using census 2000 data. Only the top ten results from this list were included. $ cat zips/98.json zipcall(["KENT, WA 98031","BELLINGHAM, WA 98226","REDMOND, WA 98052","FEDERAL WAY, WA 98023","AUBURN, WA 98023","LONGVIEW, WA 98632","EVERETT, WA 98208","SUMNER, WA 98390","LYNNWOOD, WA 98037","SEATTLE, WA 98115","VANCOUVER, WA 98682"] A file listing all zipcodes and their matching city names is also included if you'd prefer to reparse them and provide the data in your own format. There are two fields, the first being the zip code followed by a tab and then by the City and state pair. The list is sorted by zipcode. 98129 SEATTLE, WA 98131 SEATTLE, WA 98132 SEATTLE, WA 98133 SEATTLE, WA 98133 SHORELINE, WA 98134 SEATTLE, WA 98136 SEATTLE, WA