network sniffer/monitor

[mike]

I need to put a couple network sniffers on our network. The
purpose is to track and log usage. I would hope to discover
everything from "who is viewing what website" to typical
network trends about usage. I would expect having a span
port on a switch forwarding all traffic to a nic on the box
listening promiscuously. Something that put it into nice
web-viewable tables/stats would be ideal, but before I go
trying to re-invent the wheel...

It just seems like something that would be "out there".

I was wondering if there is something out there (preferably
open-source, but not necessarily a requirement) that some 
folks use and are reasonably happy with?


thx,

mike