UptimeRobot - Free Website Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible 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.
Robodash - Cron and uptime monitoring dashboard
Uptime Kuma - A fancy self-hosted monitoring tool.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring