All machines in my network are monitored using Cacti.
Cacti uses SNMP to gather various metrics from the systems, including network I/O, disk utilization and more.
I have extended Cacti to also gather WMI data from Windows machines.
All measurements are stored in RRDs (Round Robin Databases) using RRDTool.
This tool can also generate plots out of the data.
Live statistics can be found here (updated every 5 minutes, only for authenticated users).