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.
