You used to be able to find registration information on ANY COM, NET or ORG domain at
They are no longer comprehensive. Sometimes, you have to go to other registrar's WHOIS utilities, such as OpenSRS - but the WHOIS at netsol will tell you who the registrar is, and where to go for their WHOIS.
If you're doing a WHOIS on IP numbers instead of names, the starting point is
Again, this doesn't cover everything. There are three organizations that pass out IP numbers. But if you start there, you can find out where to go to do the rest of your research.
Between those two URLs and Netcraft.com, you've got a good start on researching sites.