With virtual hosting, a single IP address can host dozens of domains, and it can be interesting to find out what other domains are being hosted on the same server. Windows live search lets you use the ip: prefix to search for all the domains on a given IP address.

For example, yahoo.com resolves to 2 IP addresses: 66.94.234.13 and 216.109.112.135, but as you can see, hundreds of domains are hosted on them.

Domaintools.com provides a similar tool they call reverse IP (requires a free membership). They search through all the class A domain names and find all the ones that resolve to the same IP address. It found over 3,200 domains on the first IP address.

From the looks of it, Yahoo wants to keep other people from registering yahoo-related domain names by scooping them all up themselves.