Cacti has an extensive performance that features friendly user interface in a graphic tool that tracks monitored metric tools in a graph form. It can monitor the overall operation including the disk utilization to the fan speed and the power supply. The open source can easily track and make the data available very quickly.
The Nagios has been in the business for quite some time now and there are several open source solutions that include the same features. It is recognized by many as the old guard system and network monitoring. This open source solution is fast, reliable and complex. It has the capability to adapt to any monitoring task. Though some of you might say that the graphic interface is somewhat old school, it makes up for in power and dependability.
Last, but not the least is the Icinga. It is an offshoot of Nagios. It is considered as one of the best Nagios Alternatives when you are going to consider about its dependability. It has an alerting framework that is designed for extensive IT health care. It is closely related to Nagios while there is another version of Icinga which is the rewrite of the original one. Both are supported and if you are a Nagios user, you can easily migrate to Icinga. Since the Nagios has a tedious setup, you may want to shift to Icinga.