UptimeRobot - Free Website Uptime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
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.
RapidSpike - RapidSpike is a reliability, performance and security monitoring service for transactional websites. Using next-generation synthetic monitoring & deeper than ever before data, RapidSpike is for businesses serious about protecting their online revenue
Is it up? - A simple tool to check if a website or ip address is up or down.