Zenoss Core - Zenoss (Zenoss Core) is an open source application, server and network management platform based on...
Anturis - Anturis offers hosted server monitoring software (SaaS) for both Windows and Linux.
LANState - LANState - Network Mapper and Device Monitor
Zohno Z-Term - The Easy-to-Use Active Directory User Termination Tool
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.