Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
UptimeRobot - Free Website Uptime Monitoring
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.
Curler - Dead Simple Free Uptime Monitoring
Checkify - Super minimal, simple and inexpensive 24/7 uptime monitoring
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.