NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.

Opbeat - Application monitoring for developers

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.

AppSignal - We monitor the software that makes your customers happy.

Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.

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.