UptimeRobot - Free Website Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
WebGazer - Monitor your website's availability with WebGazer, review its performance and get instant notifications when your website goes down. No installation required and free plan available.
Cronitor - Monitor cron jobs, micro-services, daemons and almost anything else, no setup required. Easier cron troubleshooting and no more silent failures.
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.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring