As a side-project to the new AFS server upgrade project, I have implemented a SNMP-based server monitoring site using freely-available PHP-based Cacti.
Right now, statistics for each server are basic, covering CPU usage, load average, disk space consumption, and network interface traffic. In time, I will develop SNMP agents and queries that will dig deeper into what's happening on our servers such as process count (eg: monitoring the number of httpd processes on web servers, imapd/pop3d processes on the mail readers, and so on.) Also, I plan to monitor all ports on our fiber channel switches for traffic and errors.
Since every server will be running NET-SNMP, we can also utilize this as a (potential) new way to monitor servers using SNMP traps. This, however, is another project for another day.
The ultimate goal, though, is to use this as a capacity planning tool.
You can view the statistics we are collecting by clicking here.