UptimeRobot - Free Website Uptime Monitoring
Site Down - Site down outage and downtime reports from SiteDown.co
Hyperping - Cheap uptime and performance monitoring with detailed reporting and flexible alerting
Is it Down? - Is it down can check if a website is down or is it just your PC that can not access it.
UptimeRobot App - Web, SSL, ports & Cron-jobs monitoring
Is it up? - A simple tool to check if a website or ip address is up or down.