Sometimes when personalizing or translating interfaces, IP geolocation is required.
Maxmind Geolite Free
Besides their subscription based products Maxmind also provides a free IP Geolocation database set. The set is offered under the Creative Commons Attribution-ShareAlike 3.0 Unported License. The free databases maps IPv4 + IPv6 addresses and is being updated once a month. GeoLite Free Downloadable Databases
Another free set of data is provided by IP2location Lite. It is free for personal or commercial use with attribution required