OnlineOrNot - Uptime monitoring for software teams (even teams of one).
UptimeRobot - Free Website Uptime Monitoring
Site monitoring - Free open-source website monitoring software, easy to set up and use for monitoring web sites.
Uptime Kuma - A fancy self-hosted monitoring tool.
MxToolBox - All of your MX record, DNS, blacklist and SMTP diagnostics in one integrated tool.
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.