One-stop shopping for IP address and domain information
Type the information you wish to look up in the appropriate field and press the corresponding Submit button. A new browser window should then open for the return information.
InterNIC (http://www.internic.net/cgi-bin/whois) searches for mostly North-American non-goverment and non-military domain names and related information.
ARIN (http://www.arin.net/whois/arinwhois.html) is the American Registry for Internet Numbers (IP addresses).
US Military NIC (http://www.nic.mil/cgi-bin/whois) finds all kinds of US Department of Defense resources (*.mil, etc). Their own WhoIs page provides much more flexibility if you know exactly what you're looking for.
US Government NIC (http://www.nic.gov/cgi-bin/whois) finds US Government (*.gov, etc) resources (domains, e-mail, POC's), and is managed by the General Services Administration.
RIPE (http://www.ripe.net/db/whois.html) is the "Reseaux IP Europeens" or Regional Internet Registry for Europe.
APNIC (http://www.apnic.net/reg.html) is the Asia-Pacific Network Information Center, and maintains IP address allocations for that region.