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
UptimeRobot - Free Website Uptime Monitoring
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
SiteUptime - Siteuptime provides website monitoring services. Receive instant email and SMS alerts when your website becomes unavailable. View detailed uptime statistics and performance reports for your website monitors.
UPDOWNtoday - UPDOWNTODAY allows you to check whether the website is down or not.