FlowScan is a tool to monitor and graph flow information from 
Cisco, Juniper and Riverstone routers in near real-time. 

Amonst many other things, FlowScan can measure and graph traffic
for applications such as Napster.

A sample of what FlowScan can do is at: http://wwwstats.net.wisc.edu

WWW: http://net.doit.wisc.edu/~plonka/FlowScan/