I started playing with cacti (take a look here), web-based RRDtool-driven system graphing tool, and find it pretty interesting. I am hoping there are ways to add additional datapoints onto it (I passed along the script I use to drive rrdtool — the thing that makes these graphs — in case it’s at all useable.
I especially like the graph colors for CPU usage: looks like the box is on fire š
There are some built-in scripts to support data collection on some known devices or anything that runs UCD/net-snmp: OS X and FreeBSD work just fine with it for basic stuff like network monitoring and load.