Great Reverse IP lookup
If you want to see who else is sitting on your IP, WhoIs makes you sign up for a free trial (for a paid service) to get the full listing. A better option is SEOLog’s reverse IP lookup. Punch in your IP and see all your “neighbors” instantly.
I’ve noticed when I look up some gambling spammers I’ve been getting lately, it’s unable to retrieve anything for that IP. I don’t think that’s a flaw in SEOLog’s service: I think that means the spammers are using proxies or something.

IP lookup Said,
December 12, 2007 @ 3:26 pm
The IP address tools available at whatismyipaddress.com will lookup the source IP address of an email, display it on the map, and direct you to the correct WHOIS information.