I use the free Maxmind country database.

The GeoLite Country database at:

http://www.maxmind.com/app/geolitecountry)

is just fine for countries. The Ruby gem works nicely:

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/...

... and there are other APIs as well:

http://www.maxmind.com/app/api

MaxMind is the standard.

http://www.hostip.info works great for me on www.newshoured.com

I looked for a solid week earlier this year, and didn't find anything that was really free or open. Kind of disappointing.

What I needed was to be able to take an IP, and get a reasonably accurate (25 miles) lat/long out of it. If anyone knows of one that exists, please let me know...

The MaxMind GeoIP City Database includes latitude and longitude. You could buy their "Web Service" at $20 per 50,000 queries...

I guess if you were really clever you could cache it too, if you had a site with a ton of traffic.

Just hope that you get you get returning visitors then. And even then, address mappings change over time.

http://geocoder.us/. It's very accurate and you can set it up locally for speed -- it uses Perl and BerkeleyDB and the TIGER database