The Dude - MikroTik is a Latvian company which was founded in 1996 to develop routers and wireless ISP systems. MikroTik now provides hardware and software for Internet connectivity in most of the countries around the world.
cStatus - Monitor and scan a network.
Pandora FMS - Pandora FMS: Opensource monitoring software
OpenNMS - Project home page of OpenNMS - a java based network managememnt platform developed under the Open Source model
Advanced Host Monitor - HostMonitor is a highly scalable network monitoring software
SevOne - SevOne provides network managers with monitoring, troubleshooting, and performance reporting capabilities.