Whether you are running a Mac server from home or have it in a data center, cough someone is going to be interested in how much bandwidth you are using. If you want to keep an eye on it so you know before you get the bill, unhealthy Surplus Meter is a good way to do that.
Just start up the application and it will monitor everything that goes thru your ethernet port. If you have a pre-determined limit, cheap it will also help you break down how much you can use per day to stay under that limit.
SurplusMeter is free and can be found at the skoobysoft site.