Ever wonder exactly what is being sent across your network? No, I don't mean Web sites, movies, yada, yada...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 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.
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.
Microsoft Network Monitor is low-level stuff, and you really need to know networking protocols to get the most of it. But it will show you the topography of your network and give just about anyone who's interested a broad sketch of what's going on. And it does so without a heavy disk or memory penalty, annoying ads, or a rude uninstall. I like it.