Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
UptimeRobot - Free Website Uptime Monitoring
Curler - Dead Simple Free Uptime Monitoring
Pulsetic - Pulsetic makes monitoring your SaaS products simple and easy with Status badges to show the status of the site without changing pages and you will also be alerted when your site goes down.
Downtime Monkey - Downtime Monkey: website downtime alerts & uptime monitoring worldwide.
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations