Mac OS X only: Donationware application SurplusMeter monitors and records your upload and download traffic volume and offers tonnes of useful bandwidth stats, like average daily use and bandwidth allowance tracking (if your ISP has a cap, that is). Keep in mind that the application doesn't actually keep track of what traffic is from the internet and what traffic may be local network traffic (like transferring files from your desktop to your laptop), so it won't be 100% accurate if you have much network traffic on your LAN. However, if you're looking for a simple method for tracking your bandwidth, it's a handy little utility. SurplusMeter is donationware, Mac only. Windows users, check out Freemeter.