intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
One Word Domains - Database of all available one-word domains
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
Domainz.io - Premium domains, for not so premium prices
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Undeveloped - Domains you never thought were available