NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.
Zenoss Core - Zenoss (Zenoss Core) is an open source application, server and network management platform based on...
SolarWinds System Management - Performance monitoring and management for applications, Websites, servers, storage arrays and virtual infrastructure.
LANState - LANState - Network Mapper and Device Monitor
Nodeping - NodePing Server Monitoring and Web Site Monitoring. Simple but powerful Internet service monitoring.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services