Nagios - Complete monitoring and alerting for servers, switches, applications, and services
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.
System Center Operations Manager - System Center Operations Manager 2012 R2 uniquely enables customers to reduce the cost of data...
Pandora FMS - Pandora FMS: Opensource monitoring software
Zabbix - Track, record, alert and visualize performance and availability of IT resources
cStatus - Monitor and scan a network.