Network Solutions: Enhanced WHOIS Directory
Comprehensive search across multiple WHOIS databases. Includes all domains, extensions regardless of where they are registered. No need to visit multiple WHOIS Directories. Smart WHOIS Report shows the status of the domain. If a live site exists presents a thumbnail of the home page along with the website title, traffic ranking, meta descriptions and keywords, and more.

Universal Whois Search:
http://www.kessels.com/whois/

Nominet
Domain name authority for .uk domains including .co.uk. Use to find WHOIS information about .co.uk domains etc.

ICANN
Internet Corporation For Assigned Names and Numbers

InterNIC
This website has been established to provide the public information regarding Internet domain name registration services and will be updated frequently.

DNS Stuff
Free fast, advanced and comprehensive web-based DNS tools, going beyond just ping, tracert/traceroute, WHOIS and DNS lookups.

DNS Report
This site will provide you with a DNS report for your domain. A very large percentage of domains have DNS problems; this site will help you find those problems and fix them. Also, the "Mail Test" tool will help find mail delivery problems for your domain.