This is another tool I wrote about a while ago, rIP - Reverse IP is rather cool.

It's developed by a group called Crush, and they maintain a database (currently .com, .org and .net mapping every domain by IP, so they can query an IP and find all associated domains).... This is VERY handy for pen-testers and social engineers.

Example query (antionline.com):

Showing records 1 - 3 out of 3 for antionline.com. (63.146.109.212).

antionline.com antionline.org
antionline.net
An interesting query to run is google.com, you can see how many people have redirected their domains to Google (who knows why)... there are quite a few.

Either way... it's a very interesting tool. The database is available (single purchase, no updates for $20) and you can purchase a new database monthly.... so, if you have a use for it and don't want to use it online but would rather direct access to the database you're looking at $240/year.