If you are a customer of an Internet Service Provider (ISP) who is monitoring your bandwidth usage, a useful tool for you is a bandwidth meter. While most of you out there will probably never run into issues with bandwidth limitations, inevitably some of you will. To help watch your usage, check out NetMeter.
NetMeter is a free bandwidth monitoring tool which watches both your upload and download usage. There are lots of options you can configure such as the color and font scheme as well as which network cards you want to watch (in the event you have more than one). Another handy feature is NetMeter can project your anticipated usage which may be attractive for those with limations.
For the most part, this utility is a good tool to have for those concerned about their net usage. Power users may find this an interesting toy to play with as well.