The best free tool for bandwidth monitoring I know of is
mrtg. I found an OS X binary for it
here. Follow the link on that page to download the binary for the gd package that's used to generate the graphs.
mrtg uses the SNMP protocol to query routers about their traffic loads. I've used it with commercial routers like Ciscos and with home-made Linux routers. If you want to monitor the traffic on a specific machine, you'll probably need to run the snmpd daemon so mrtg can query the machine it's running on. I would imagine there's an snmpd daemon already available for Macs since it's a very common protocol.
I did a quick search at sourceforge.net and didn't see anything that ran on a Macintosh. It might be possible for someone to port code written for Linux, but I'm not that someone.
