Geocoding is (according to Wikipedia), "the process of assigning geographic coordinates (e.g. latitude-longitude) to street addresses, as well as other points and features. With geographic coordinates, the features can then be mapped and entered into Geographic Information Systems."
geocoder.us, a free US geocoder to convert addresses into latitutude and longitude
From their site: "We encourage you to write code using our web services API for non-commercial purposes! You can read our developer documentation, and take a look at some example SOAP, XML-RPC, REST-ful RDF, and NEW REST-ful CSV (plain text) clients. Please drop an email to services@geocoder.us if you plan to build our web services into your application, so that we can keep you informed of service updates or changes. We intend to make every effort to keep the geocoder running stably, but this service is provided with no warranty or guarantee of availability!"
A note from the Geowanking mailing list indicates how to use geocoder.us with a particular XML-RPC package for PHP
Yahoo Maps Web Services has a Geocoding API.
Google Maps API. "A free beta service, available for any web site that is free to consumers. Google retains the right to put advertising on the map in the future."