Microsoft’s search engine Bing has an interesting operator which can give you the list of all domains hosted on any given IP.
First thing is to get the IP of a domain, e.g. by entering ping www.example.com into cmd.exe on your windows box and host www.example.com on your *nix box. Copy the IP into Bing and precede it with “ip:”, as in ip:188.8.131.52.
The search will now show other domains than example.com hosted on the server.