Good old MRTG. This is the quickest, most flexible network polling system that I have ever played with.
This software is priceless when troubleshooting or monitoring equipment after a build. I have run this from old laptops with only 2 GB or RAM, 1 Ghz processor and 80 GB hard drive with no issue. You can graph any numeric value via SNMP or logs.
In this video, I show you how to get this up and running in just a few steps.