Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Simple Ops - Simplify website performance and uptime monitoring with alerting, ssl check, chrome ux metrics, multi locations
Better Uptime - We call you when your website goes down