Host monitoring
Host status: up/down, cpu, load, swap, processes, disk usage, and other plugins...
Network monitoring: nfs, ldap, ntp, httpd, dns, smtp, and more...
Other Monitoring
Plugins may easily be written for host, network and other monitoring
Alerts (email, pager, sms text messages, IM, audio)
You may wish to view a snapshot of Nagios on my network