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.
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
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
Monitor Buddy - Simple & cheap uptime monitoring service