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In Australia, the Postal Service can supply a data file or xml integration of all combinations of City, Zip Code and State. I would assume that this is available in other countries also.
Salesforce should subscribe to this data on a global scale on behalf of its users, and provide an automatic validation in all Address Fields, including functionality to auto-populate any missing Fields.

Imagine this: If a city field is blank when a record is saved, an error message and a dropdown box will appear with the available cities that match the zip code entered - based on the Data supplied to Salesforce.

OR

A city is changed but not the zip code or country and it now doesn't match - show drop down boxes with the available correct options.

Obviously this would provide a massive benefit to users in terms of data integrity, and any costs when spread across all Salesforce Users would be negligible - and ALL Salesforce Users would benefit.

Once implemented, it would take little effort for a Salesforce rep in each country to get updated date as required. It could even be managed by Users somehow.

Imagine how many Salesforce instances have this as custom built functionality, all created differently with varying degree's of complexity. If this functionality was standardised in Salesforce, it would create a brilliant streamlined model that would be a great selling point to new customers.