location autocomplete

Google Maps API v3 Geocoder Tool
http://gmaps-samples-v3.googlecode.com/svn/trunk/geocoder/v3-geocoder-tool.html#q%3D2500%20KYUSTENDIL%26country%3DBG
http://groups.google.com/group/google-maps-js-api-v3/browse_thread/thread/822b416df3d19bfa

jQuery plugin to autocomplete location using Google Maps v3 API and Google Static Maps v2 API
http://code.google.com/p/geo-autocomplete/
http://geo-autocomplete.googlecode.com/svn/trunk/demo/ui.demo.html

reverse geocoding demo: click on map, get detailed address
http://demos.projectx.co.nz/gmaps3/reverse_geocoder.html

Google Maps Javascript API V3 documentation
http://code.google.com/apis/maps/documentation/javascript/services.html

UK-specific
http://www.oak-wood.co.uk/2010/04/postcodes-freed/
http://www.uk-postcodes.com/
http://www.tomanthony.co.uk/blog/geocoding-uk-postcodes-with-google-map-api/

databases:
http://www.geopostcodes.com/resources
http://www.upu.int/en/resources/postcodes/universal-postcoder-database.html

lookup and validation services
http://www.addressdoctor.com/lookup/default.aspx?lang=en
http://www.uniserv.com/en/products/data-quality-functions-features/address-validation-data-capture.php