In the face of internet service providers like Comcast instituting bandwidth-capping, the Simple Help weblog details how to use a router running the open-source DD-WRT firmware to monitor your bandwidth. It's actually very simple to do, requiring no work on your part aside from installing DD-WRT on your router. DD-WRT automatically tracks bandwidth, so from there it's a matter of knowing where to look. If you're running the user-friendly Tomato firmware (we also showed you how to install Tomato), you can easily access your daily, weekly, or monthly bandwidth as well.
Like DD-WRT, Tomato automatically tracks bandwidth usage for you, so all you need to know is where to look. Just log into the Tomato interface, and then click on the Monthly link under Bandwidth in the sidebar (or, if you're using a default configuration, just follow this link). You'll get a simple table displaying your bandwidth stats for the month. Tomato also displays bandwidth use by week, day, and even in real-time if you're interested.
I've used DD-WRT and Tomato exclusively over the past few years, so I'm not really sure what the status is for bandwidth monitoring on most default router firmware. If your router supports bandwidth monitoring, let's hear about it in the comments.