Ethereal: A Network Protocol Analyzer
Above is a link to ethereal. It is an excellent tool to analyze the packets sent and received by your machine. It is very interesting to look at and extremely helpful to diagnosis of problems in general.
There is a slight learning curve for how to use it well. But if you are of the type that would be interested in this kind of thing, I trust you are also of the type that can use it well.
For instance, you can see all the traffic on your local lan and what your computers are saying to each other. You can see how your router calls out to your machines to ensure they are still on the network. Lots of other cool things.
You can see when you start up certain software that they are trying to "call home" which many people find they do not like at all.
Play around with it some and you may be glad you have a new tool at your disposal. You could use it to capture all packets sent and received, for many different purposes, from any specific adapter. Perhaps you want to see what your computers are doing while you away or while another is using? Alright, that's a little shady though, heh heh.