sFlowTrend is a free, graphical, sFlow collector, plotting top talkers and interface counters over time. sFlow is a monitoring technology, built into many different vendor's switches, including: Foundry, Extreme, Force10, HP Procurve, Hitachi, Allied Telesis, AlaxalA and Comtec.
sFlowTrend can be downloaded here:
formatting link
If you want to find out more about sFlow, see:
formatting link
sFlow.org lists devices that support sFlow:
formatting link