Windows: If you're worried that some application may be eating into your bandwidth in the background, or you just want to keep closer tabs on the bandwidth your computer is using, NetSpeedMonitor is a handy systray utility that lives in your taskbar. It shows you active upload and download speeds, which applications are using your network connection and what their endpoints are.
There are plenty of tools that will tap your network connection so you can keep an eye on it, but NetSpeedMonitor doesn't require additional drivers, and it integrates perfectly with Windows, running quietly in the background while you do other things. Your up/down speeds are shown in the taskbar, and you can right-click on the app at any time to see which running programs are using your connection -- useful for rooting out a rogue app that's sending data even when everything is supposed to be shut down.
The app is also useful if you're worried about how much bandwidth you're using. Even though it can only report on one connection on one computer, if you do most of your browsing on that one system, it can give you an idea of how much data you're consuming every day, week or month.
NetSpeedMonitor isn't exactly new, but it is useful. Plus, it's completely free. It's available for Windows XP, Windows Vista, Server 2003 and Windows 7, and it comes in both 32-bit and 64-bit flavours. Used it before? Have an alternative? Let us know your preferred app in the comments below.