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.
SmartBear - For a world in constant flux, quality matters more than ever. So wherever you're going, we'll help you get there.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Paessler - PRTG Network Monitor is a network monitoring and infrastructure management tool. It lets network administrators monitor devices such as access points, routers and switches. Read more about PRTG.
Better Uptime - We call you when your website goes down
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.