UptimeRobot - Free Website Uptime Monitoring
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
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.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Uptime Kuma - A fancy self-hosted monitoring tool.
Monitornator - Server monitoring made simple