Nagios - Complete monitoring and alerting for servers, switches, applications, and services

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

Gitwin - Enable your Windows system as a powerful and secure Git repository. Supports ssh, http(s) transports. Gitweb with highlighter and gravatar support available.

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.

Zenoss Core - Zenoss (Zenoss Core) is an open source application, server and network management platform based on...

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