NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.
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.
Dynatrace Ruxit - All-in-One Application Performance Monitoring
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
DareBoost - DareBoost is an online tool to test, analyze and monitor your website: speed, SEO, quality, security. All you need for website optimization in a single service.