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.
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.
LightStep - We deliver insights that put organizations back in control of their complex software apps.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
PingPing - A simple tool to monitor your website and SSL
Better Uptime - We call you when your website goes down