Supervision and metrics are quite well covered by open source solutions, with many quality products available.  Nagios in particular enjoys a very strong reputation.
Supervision tools are used to monitor networks, machines, services and more. The functionalities they provide include views of the status of supervised services and machines, metrics, reporting, access to supervision events, and advanced user and ACL management, but also dependencies, escalations, service and host templates, active and passive monitoring support, etc.
Six open source solutions have been evaluated for this category.

Solution	Views	Global Rating
		4.0 5
		4.3 5
		4.4 5
		4.0 5
		3.2 5
		4.0 5
MRTG		No rating
Zenoss		4.9 5
		4.2 5
Shinken		2.7 5