Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
intoDNS - IntoDNS checks the health and configuration and provides DNS report and mail servers report.
UptimeRobot - Free Website Uptime Monitoring
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
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.