DomainTools is without a doubt the absolute leader in domain tool services. Founded by renowned domainer Jay Westerdal, the site offers a basic whois search, whois histories for domain names, cross-IP lookups, a Name Server Spy, a typo-generator, and dozens of other useful domain tools.
In addition to the popular domain tools, the site also offers a variety of DNS tools, such as ping and traceroute. Powerful search features include a domain suggestion search which has provided over 10 billion suggestions per year, a domain auction search, and a domain for sale search.
Many of their tools are free, although some have varying usage restrictions that require a subscription. The subscription system follows a point system whereby the more money you pay, the more points you get, and in turn the more tools you can use. Subscriptions currently range from $15.00 to $30.00 per month.