Is It Down Right Now? - Network & Admin
UptimeRobot - Free Website Uptime Monitoring
Is it up? - A simple tool to check if a website or ip address is up or down.
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 Down? - Is it down can check if a website is down or is it just your PC that can not access it.
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.