Atelier Web IP Locator is a simple tool which will tell you at a click where in the world an IP address is located, returning the city, country, ISP and the starting and ending IP address for that block.
You can do this from lots of different online services, of course. However, IP Locator allows you to search for multiple IP addresses in a click; and the authors say it uses "the World's biggest IP geolocation database" for accuracy; and it includes a freely distributable Automation Server and .Net Library which means developers can use it within their applications or on web sites.
While you can search for several IP addresses in one go, the free version limits this by giving you 200 search credits for the first 30 days, with 50 further credits being provided every subsequent 30 days.
But if that's not enough then you can order a commercial version which provides search allowances ranging from 10,000 ($19.95) to 10,000,000 ($699.95) - see the website for more.
A fast, easy-to-use and accurate IP geolocation system