network usage monitoring tools

[Shawn L Johnston]

I'm looking to add some sort of network monitoring tool to our LAN at 
the office.

I want to be able to see how much of our bandwidth is being used and 
from which systems. I've taken a quick look at Sniffer Pro from Network 
Associates, but I'd like to go with an opensource tool for this instead.

Any recommendations, places to do further reading?

Many thanks,

Shawn