http://www.networksolutions.com/en_US/whois/index.jhtml
It's called a "WHOIS" search.
Say the company you want to know about is www.computersite.com
Put "computersite.com" (less the quotes) in the search box and hit Go. Next page that comes up will show what you're lookin' for.
Tom
Name Servers:
ns1.interspeed.com
ns2.interspeed.com
Current Registrar: ENOM, INC.
the hosts site is more likely to be http://interspeed.com/ since that is where the nameservers are, although you could always ping the domain name, and go to the IP address to see if that goes to a hosts site