UptimeRobot - Free Website Uptime Monitoring
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
UptimeRobot App - Web, SSL, ports & Cron-jobs 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.
Pager Team - Get system outage notifications instantly
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations