NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
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.
Server Monitoring - A server monitor is a tool that monitors your server and website on close intervals and reports to you if it's inaccessible.
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.
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
UptimeRobot - Free Website Uptime Monitoring