UptimeRobot - Free Website Uptime Monitoring
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Uptime Kuma - A fancy self-hosted monitoring tool.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services