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.
Lumturio - Lumturio is a tool to monitor websites built on Drupal and WordPress.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Turbo Monitor - The fastest uptime monitoring service currently available. Receive free e-mail or notification alerts of website failures instantly, and protect your website.
Better Uptime - We call you when your website goes down
Pingometer - Pingometer is a service that monitors the uptime, downtime, and performance of websites. Get 24/7 monitoring - sign up for your FREE account today!