myIPneighbours reverse IP check
Found via Digg:
myIPneighbors is a useful little site which allows you to see which domains are hosted on a particular IP address. There are several sites out there which already offer this functionality, but many of them charge for access or limit you to a very small number of queries. The data on the site seems to be a little bit out of date and doesn’t always match up (e.g. if you search for rixort.com you’ll get the correct IP address, but do a reverse lookup on that IP and you won’t get any results), but it’s an interesting tool to play around with.