Google has a geocoding API which seems to work pretty well for most of the locations that they have Google Maps data for.
http://googlemapsapi.blogspot.com/2006/06/geocoding-at-last.html
They provide online geocoding (via JavaScript):
http://code.google.com/apis/maps/documentation/services.html#Geocoding
Or backend geocoding (via an HTTP request):
http://code.google.com/apis/maps/documentation/services.html#Geocoding_Direct
The data is usually the same used by Google Maps itself. (note that there are some exceptions to this, such as the UK or Israel, where the data is from a different source and of slightly reduced quality)