NetTraffic is a bandwidth monitor which will help you to keep track of exactly how your internet connection is being used.
This starts very simply, with the program displaying an animated icon in your system tray which flashes as information is uploaded or downloaded. Hover your mouse cursor over this and a tooltip displays the current upload and download speeds. (Initially this is for all network traffic, although you can ask it to monitor specific interfaces from the program's settings dialog.)
If you need more, right-click the system tray icon, click Show and a small graph will appear which charts network activity over time. This will stay on top of other windows, conveniently, and can also be resized if you'd like more detail.
Right-click the chart or system tray icon, select Statistics, and you'll be presented with tables listing the total and average network traffic for the last hour, day, month and year. And it'll even try to predict your future bandwidth needs for the same periods of time (this didn't look too accurate in our tests, but the figures should presumably become more reliable as NetTraffic has longer to sample your typical network use).
There's also an option to zoom in on a particular period of time in your network history, and display charts of your network use. So if someone boosted your figures with a massive late-night downloading session, say, you'll find it recorded here.
And the program even tracks and displays the distance travelled by your mouse cursor over the same period. We're not entirely sure why... But it's here, if you need it.
The only minor issue we could find with NetTraffic is its resource use, which was a little higher than we'd like (20-40MB).
This isn't enough to make the slightest difference to most people, though, and so if you need to track your network bandwidth then you should give NetTraffic a try: it's easy to use, provides plenty of information, and offers a host of configuration options and settings to ensure it's working just as you'd like.