Nitpicker’s corner: so the title it not 100% correct, since I’m focusing more on the question “what websites are hosted on this server” rather than “what DNS names is this server known under”.

Here are some possible ways to answer this question:

• Do an actual reverse DNS lookup. The problem is that there isn’t actually any technical obligation to register the IP in the reverse lookup zone, and many administrators don’t do it, because it is an extra hassle.
• You can use services like Domaintools, but you need a (for-pay) subscription to see the complete list (without an accout you get the first 2 domains I think)
• You can use DNSLogger instances – originally here, now moved here – credit goes to my dear friend for having better memory than me and remembering these sites. What this service does, is that it sits on a large pipe, captures DNS responses, learns the name <-> IP associations from there and provides it as a searchable database.

And now here is a possibility I’ve learned about recently from Johann Burkard’s blog: the Microsoft Live search engine supports the “ip:” operator, giving you all the sites which were indexed on the given IP address.