http://whois.net/ is the one I use most of the time. You can search by domain name and get the info the registrant supplied when they set up the domain name. ~Peach
I find whois.net works about 2/3 of the time, and for those where it doesn't, www.betterwhois.com deals with the majority of them. I generally try betterwhois first. Neither of them work for .edu domains; you can use http://whois.educause.net/ for that.