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.
Pandora FMS - Pandora FMS: Opensource monitoring software
cStatus - Monitor and scan a network.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Trafikito - Monitor CPU, RAM... or output of any command and make API call on event, default is email messages.
NetXMS - NetXMS is an enterprise grade multi-platform open source network management and monitoring system.