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.
Site Down - Site down outage and downtime reports from SiteDown.co
Upzilla.co - Upzilla.co alerts you when your websites, services, or applications have some issues with availability or performance. Quick and simple setup, multiple notification methods, and it's FREE.
Is it up? - A simple tool to check if a website or ip address is up or down.
UptimeRobot - Free Website Uptime Monitoring
downinspector.com - Find out if a website is down for everyone or just you.