Via lifehacker.com:
If you have a Raspberry Pi and one of those surge protectors that also protects your network cables from voltage fluctuations, you have most of the tools you need to build a cheap network observation device, as long as you’re not afraid to use a soldering iron. When you’re finished, you’ll have the perfect addition to your pen-testing toolkit.
To make this work, you’ll need a Raspberry Pi, any surge protector with RJ-45 protection (the folks at Gnurds used this one), a low-profile SD card adapter for the Raspberry Pi, a few cables, a soldering iron, and some electrical tape. You can get a full parts list (and walkthrough of the build) at the link below.
When you’re finished and plug everything in, you’ll end up with a surge protector that looks completely inconspicuous, but can capture any traffic to and from the system it’s connected to. You’ll have to come back to pick up the SD card periodically, but you could fix that by adding a Wi-Fi or 3G radio to the sniffer, drive up outside, log in, download your data, and drive away (of course, broadcasting wirelessly will increase your odds of detection and make the build bigger, so there are drawbacks.)
Featured Adafruit Products
Low-profile microSD card adapter for Raspberry Pi – Make your Pi a little slimmer with this microSD card adapter board. It slides in where the SD card goes but is half the length. Pop in a microSD card for a sleeker machine. The microSD card holder is a push-push type so you can push on the edge that sticks out to remove the card when necessary. (read more)
Each Friday is PiDay here at Adafruit, be sure to check out our posts, tutorials and new Raspberry Pi related products. Have you tried the new “Adafruit Raspberry Pi Educational Linux Distro”? It’s our tweaked distribution for teaching electronics using the Raspberry Pi. But wait, there’s more! Try our new Raspberry Pi WebIDE! The easiest way to learn programming on a Raspberry Pi.
We now have Raspberry Pi Model B with 512MB RAM in stock and shipping now!
The network cable is most probably connected to the internet (that’s what most people use the network for…) so you can just sent the data to a webserver or so…