Or you could write your own, its very easy to convert the ARIN, APNIC,  
RIPENCC, LACNIC, from these assigned files
ftp://ftp.arin.net/pub/stats/arin/delegated-arin-latest
ftp://ftp.apnic.net/pub/apnic/stats/apnic/assigned-apnic-latest
ftp://ftp.ripe.net/ripe/stats/delegated-ripencc-latest
ftp://ftp.lacnic.net/pub/stats/lacnic/delegated-lacnic-latest

These update daily, you can store them in your database and then do a  
search on them.

To store the IP Addresses convert them to decimal from dot-quad as  
explained here: http://www.codeproject.com/useritems/NetCalc.asp

However if your after something more specific than resolving to a country,  
you can just use maxmind.com or another similar service if your willing to  
pay.

- Mark