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.
DropTxt.me - Get a text message straight to your phone when a Drupal security announcement is released
UptimeRobot - Free Website Uptime Monitoring
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!
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.