SPF Toolbox - SPFtoolbox is a Javascript and PHP app to look up DNS records such as SPF, MX, Whois, and more
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
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.
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.