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.
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
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.