[staunch]

The Maxmind GeoIP database is what most people are using. It's quite accurate, has good libraries for many languages, but it's proprietary and costs money.

(Starting an open source competitor is on my list of fun projects to do)

[tcwc]

What about their Open Source version - http://www.maxmind.com/app/geolitecountry ?

I haven't started using it yet, but they claim it offers only slightly less accuracy with access to all of their libraries.