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.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Checkify - Super minimal, simple and inexpensive 24/7 uptime monitoring
Pager Team - Get system outage notifications instantly