geoPlugin - the original and largest free IP geolocation web service API
geoplugin.com offers multiple geocoding APIs, including IP geolocation, currency conversion, reverse geocoding since 2004 from its web service API entry point geoplugin.net.
We were the first free IP geolocation web service on the net and handle around 1/10 of the global traffic that google.com handles (live stats available on geoplugin.com) each and every day.
All this for free (with a free limit of 120 requests a minute).
No API key or registration required to use the API.