Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
One Word Domains - Database of all available one-word domains
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Domainz.io - Premium domains, for not so premium prices
SolveDNS - SolveDNS works to check your name servers, a records, and mail servers, SOA records for errors, consistency, efficiency and speed using several DNS tests.
Undeveloped - Domains you never thought were available