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.
Uptime.com - Everything you require for unified availability monitoring Simple & intuitive industry leading Enterprise-grade features delivered at a fair price, that are continuously improving.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Upzilla.co - Upzilla.co alerts you when your websites, services, or applications have some issues with availability or performance. Quick and simple setup, multiple notification methods, and it's FREE.
Better Uptime - We call you when your website goes down
Pingnik - Web Monitoring around the world