Here's a great tip via Google Blogoscoped (even though this tip has to do with Bing). Want to find out if any other domains are associated with your domain's IP address? Bing allows you to find out.
Use a site that looks up the IP address of a domain such as IP-Address.com. Then plug the IP address into Bing, preceded with "ip:".
To try it out, I used a local tv site, WRAL.com. Their IP is 22.214.171.124. You can see other sites share the same IP. (The sites are associated with sites that the parent company of WRAL.com owns.)
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