FlowScan is a freely-available network traffic analysis and visualization tool which uses IP flow data to provide a continuous near real-time view of a network’s IP traffic. FlowScan is a tool to monitor and graph flow information from Cisco and Riverstone routers in near real-time. You can also use FlowScan with flow information generated by programs like fprobe.

Flowscan in action:

Here is a sample Graphical output (take from: http://wwwstats.net.wisc.edu/)

For more information about flowscan check out:
Official Website: http://net.doit.wisc.edu/~plonka/FlowScan/
Papers Written on Flowscan: http://www.usenix.org/events/lisa2000/plonka.html
http://net.doit.wisc.edu/~plonka/lisa/FlowScan/