Note: The Google geocoder is not optimal for all/many situations, particularly those requiring massive amounts of geocoding for a preset dataset.

There are many alternative APIs and services out there, depending on the region desired.

Geocoding is the process of assigning geographic identifiers (e.g., codes or geographic coordinates expressed as latitude-longitude) to map features and other data records, such as street addresses. You can also geocode media, for example where a picture was taken, IP Addresses, and anything that has a geographic component. With geographic coordinates, the features can then be mapped and entered into Geographic Information Systems.

http://en.wikipedia.org/wiki/Geocoding

Official Post:

http://googlemapsapi.blogspot.com/2007/07/uk-geocoding-now-available-in-maps-api.html

New! Google Android Geocoder (& Reverse Geocoder) in Release Candidate 14 (13th Feb 2008)

http://code.google.com/android/reference/android/location/Geocoder.html

http://android-developers.blogspot.com/2008/02/android-sdk-m5-rc14-now-available.html

Geocoding  - From an address match to co-ordinates (Lat/Lon)

• Jetkey SQL Ingetrated Geocoder: Integrates both Google's & Yahoo's geocoders into MS SQL Server 2005 & 2008 databases. Allows you to gecode addresses directly within SQL databases without relying on external scripts. 
• Where2GetIt REST API Geocoding and reverse geocoding (also includes driving directions and proximity search)
  
• iMaptools Geocoder :: an excellent geocoding solution with US coverage
• http://www.geonames.org/export/ :: Worldwide Placename and Zip Code Geocoder
• Yahoo Geocoder (useable only for Yahoo Maps according to Yahoo license agreement)
• InfoSports :: Manual Geocoder
• BatchGeocode.com :: locates multiple addresses using Yahoo's Geocoding API
  
• GPS Visualizer's Quick Geocoder :: locates a single address using Yahoo or Google and shows all information returned from the API
  
• GPS Visualizer's Easy Batch Geocoder :: locates multiple addresses using Yahoo; preserves structure of formatted tabular input data, but also allows unstructured input
• placemarks.net :: locates multiple addresses using Google's Geocoding API. Located adresses are downloadable as KML files. Page content is also available in german.
  
• Emad :: Supports UK Addresses
• Offentlig Information Online :: Geocodes addresses in Denmark
• Maps.Huge.Info :: Geocoder.US/Tiger Geocoder - 2005SE Database
• nearby.org.uk API :: UK Sector Postcode and US Zip code Geocoder
• IP Address Geolocation :: retrieve the latitude and longitude of the Internet users using IP address
• IP geocoding :: retrieve the lat/long of the Internet Provider
• Map24 - Mapsolute Developer Network -> Step-by-Step Description - Geocoding (Free Map24 AJAX Application Key required, 10,000 transactions limit per day)
• Multimap Address and Postcode Geocoder - Using MultiMap API Geocoder (v1.2) - Supports UK Addresses/Postcodes and Other Countries
• Singapore Postal Code Location Finder. Beta Version 1.0
• Postal Code Geocoder :: Reverse Look Up United States, Canada and Mexico Postal Code to Latitude and Longitude 
• GeoBase :: Worldwide geocoding.
• World Geocoder and Reverse Geocoder - Supports Russia 
• opengeocoding.org - a community based approach to provide geocoding for all countries, especially for developping countries where  commmercial solutions are not available.
• Irish Address geocoding Using Yahoo to geocode Irish addresses in google maps
• Geokodierung.net - Worldwide Placename, Zip Code Geocoder, dragable pointer
• iBegin Geocoder  - Can geocode United States or Canada
  
• Japanese Address Geocode in English. Geocode Japanese addresses in English.
• European Geocoder including the UK using ViaMichelin Geocoder
• Universal Geocoder/Reverse Geocoder (24 Countries including UK)
• Map Channels Geocoding  - batch and interactive geocoding using the Google Geocoder, includes a comparison option to compare geocodes with existing co-ordinates.
  

Reverse Geocoding - from co-ordinates (Lat/Lon) to an address match

• Where2GetIt REST API Reverse Geocoding and Geocoding (also includes driving directions and proximity search)
  
• iMaptools.com Reverse Geocoder :: Reverse Geocoder for Location-Based Services
• ASK City :: Manual (use the blue icon - supports UK requests) 
• Geonames.org Reverse Geocoding :: US Address, and Worldwide Placename and Zip Code Reverse Geocoder
  
• Maps.Huge.Info :: USPS AIS Zip+4 Based Reverse Geocoder - US and Possessions
  
• USNaviguide :: Tiger/Line Based Reverse Geocoder - US and Possessions
• nearby.org.uk API :: UK Sector Postcode and US Zip code Reverse Geocoder 
• iBegin Geocoder  - Reverse geocoding for United States or Canada
  
• GeoBase :: World wide high speed reverse geocoder for client and server based application
  
• FreeReverseGeo :: Free Reverse Geocoding for US and Possessions, with Google Maps API Example
• Reverse Geocoding example using Google Maps API only :: Reverse Geocoding using Google Maps API Driving Directions (24 Countries) - Documentation
• The API Geocoder will (unoffically) Reverse Geocode to Country level now, see message
• Reverse geocoder :: Reverse geocoding HTTP web service (US - street level, worldwide - city level)
  

 Geocoding Helper Libraries bvcvb

• GeoKit is a Ruby on Rails plugin which provides a uniform Ruby API to multiple geocoders (currently, Google, Yahoo, Geocoder.us, and Geocoder.ca). It also provides IP-based geolocation, and a configurable fail-over mechanism for the geocoders.
• WebRPC allows you to play with the google geocoder online, generate client code in .NEt, Java, JS and PHP and monitor service availability and performance