UptimeRobot - Free Website Uptime Monitoring
Dynatrace - Cloud-based quality testing, performance monitoring and analytics for mobile apps and websites. Get started with Keynote today!
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.
Site24x7 - Site24x7 offers both free & paid website monitoring services. Monitor websites remotely and receive instant email/sms alerts if your website becomes unavailable. View uptime & performance graphs of your website monitors.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Catchpoint - Catch issues before they impact your business