Microsoft Network Monitor 3.4 is a free utility that let's you monitor data traffic across your network.
Do you ever wonder exactly what is being sent across your network? No, I don't mean websites, movies, and so on, I mean in the detailed sense: the packets that comprise the lowest level of network traffic and which programs are sending them. If the answer is 'yes', Network Monitor 3.4 - a free utility from Microsoft - is all you need to find out what's flowing back and forth.
There are certainly network monitors/analyzers with more bells, whistles, and graphics such as The Dude and Spiceworks, but Microsoft Network Monitor 3.4 is lean and mean. The disk footprint is minimal and CPU usage miniscule, but it still does the job. If there's a packet it doesn't understand, there's probably a free third-party parser that you can install to handle it.
Microsoft Network Monitor 3.4 works in real time, but only after you've started or loaded a capture of information. The idea is to capture data, then stop and analyze it. You can save and open the info at any time. You may also download and run one of Microsoft's "expert" analyzing tools that will tell you the heaviest users, decrypt encrypted packets, etc.