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 up? - A simple tool to check if a website or ip address is up or down.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
downinspector.com - Find out if a website is down for everyone or just you.