Suprvise - Know instantly when your sites are down.
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.
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.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Spike - Spike solves businesses' communication chaos by bringing together email, team chat, collaborative docs, and meetings, into one feed.
Better Uptime - We call you when your website goes down