Here is the full article that gives more of a description of the tools. I have made a short list of the tools for your quick browsing pleasure.
http://sixrevisions.com/tools/10-free-server-network-monitoring-tools-that-kick-ass/
10 Free Server & Network Monitoring Tools short list.
1. Monit
2. Ganglia
3. Munin
4. Cacti
5. Nagios
6. Zabbix
7. Observer
8. Zenoss
9. Collectd
10. Argus
I personally use Cacti, but it is a pain to set up. I would recommend downloading a vms of it. It is a pain to set up all the necessary tools to work together unless you have a clean system to start with.