Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
M/Monit - Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
Pandora FMS - Pandora FMS: Opensource monitoring software
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
OpenNMS - Project home page of OpenNMS - a java based network managememnt platform developed under the Open Source model