What kind of Cisco switch do you have? If you have a semi-high end one that has the huge expansion slots, you can get something known as a "Supervisor" from Cisco. That tracks every port.
Other then that...you log in through the console of the switch and in user mode type something such as (IOS version pending):
show interface fastethernet 0/1 bandwidth
And it'll show you how much bandwidth has been used (in Bps, I believe). If you want a more precise command I can give it to you when I get to the lab on Monday, but I'll tell you right now that that probally isn't what you're looking for because it's a total count and I don't believe you can reset it.
I use mrtg to monitor co-location boxes on my site. Your switch needs to have its own ip-adress set up, and basic snmp support, and the workstation you run mrtg from needs access to read the snmp stuff from the switch (usually no problem)... mrtg comes with an automatic configuration tool however that can figure out mostly everything for you. Then set up mrtg to run every 5 minutes through cron and you get nice web pages with graphs of everybodys network usage based on the port they're connected to.