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.
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
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.
RapidSpike - RapidSpike is an uptime and performance monitoring service for websites and web applications.