Free the postcode

Google provide an API for the UK that basically gives you a lot of the 'clever' stuff that Maps provides in terms of finding an address which can be quite useful. Not sure if this is available in other countries...

There are various different query APIs available. Google maintains a useful list:

  http://groups.google.com/group/Google-Maps-API/web/resources-non-google-geocoders

One of the major problems is that the use of the data returned by many of these services is restricted. There are a couple of entirely free solutions. For example, I put together Fetegeo http://fetegeo.org/ to show that it is possible to have fairly decent search facilities with solely free data (though the currently available data sources all have various limitations). I keep hoping that someone will take Fetegeo and plug-in data sources like Openstreetmap.