StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot - Free Website Uptime Monitoring
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.
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.
HetrixTools - FREE Uptime Monitoring + Server Monitoring + Status Page + Blacklist Monitoring!
WatchMouse - Website monitoring service for web server performance measurement.