Prometheus - An open-source systems monitoring and alerting toolkit.
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.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Pandora FMS - Pandora FMS: Opensource monitoring software
StatsD - Simple daemon for easy stats aggregation.
cStatus - Monitor and scan a network.