Most internet service providers offer some form of tracking service to see how much data you've used in a month, but that's usually browser-based. If you'd prefer to keep track of your usage via a desktop gadget and you're running Windows, then the Unique Windows Gadgets site probably has a tool that will fit the bill.
The site includes gadgets for tracking usage on BigPond, Internode, iiNet, Westnet, Eftel and Supernerd. I tested out the Internode gadget on my home account, and it worked fine. You can set the interval for it to update usage data, but other than that there isn't much to configure. The gadgets are free and work on Vista or Windows 7. Thanks Jayden for the pointer!