Library to find the country that any IP address originates from

GeoIP is a C library that enables the user to find the country that any
IP address or hostname originates from.  It uses a file based database.
 This database simply contains IP blocks as keys, and countries as
values.  This database should be more complete and accurate than using
reverse DNS lookups.

MaxMind offers a service where you can have your database updated
automically each month.