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.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Dnsstuff - DNSstuff offers DNS tools, Network tools, Email tools, DNS reporting and IP information gathering.
Better Uptime - We call you when your website goes down
Misk.com DNS Lookup - Powerful domain name and ip address tools for DNS Lookup (nslookup), Supports Reverse DNS, FCrDNS...