Is It Down Right Now? - Network & Admin
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.
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
Better Uptime - We call you when your website goes down