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Orion NPM is pricey. Worth it, but often out of reach for small budgets. In its full-sensor glory, Paessler PRTG is in a similar vein. The difference is that you can use a 10-sensor version of PRTG for free (and then grow into more). If one of those sensors is NetFlow or the port-sniffer module, then you can have the same "top user" graphs as in NPM, but without the big price tag. (SolarWinds has also recently introduced a free NetFlow tool as a step-stone to NPM, but not sure if it as full-featured as PRTG's offering.) For my beginner's intro to network monitoring, see http://siliconexus.com/blog/2011/10/an-overview-of-network-monitoring/.