Portable Hardware Network Sniffer

They have all of those portable hardware based key loggers out there, why not something for network traffic? Something inconspicuous that you could just plug and let sit for a few days. 

I'm searching around right now but haven't found much yet.

Here's a cool site where the guy makes his own Ethernet tap:

http://www.enigmacurry.com/articles/building-an-ethernet-tap/