StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
UptimeRobot - Free Website Uptime Monitoring
Uptrends - Uptrends is a website and mobile app that allows you to monitor your website. You can use it to both monitor the security and performance of your website as well as the activity on your site. Read more about Uptrends.
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.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
eZ Server Monitor - eZ Server Monitor is a script for monitoring Linux servers.