You can plug a mini-hub in line and use that as a tap point to monitor the stream. Up side is its cheap and easy. Down side is you have to drop to half duplex. Not a problem in most situations but in some the drop in performance can be an issue.

Don't throw out your old hubs. It's hard to find a 10/100 hub for sale any more. Even the cheap consumer devices are switches. I picked up a $25 hub at Fry's a few weeks ago just in case I ever wanted to casually snoop some traffic. But Fry's is sold out. The netoptics.com link posted was "priceless". I'm always wary of simple products that are expensive enough to have a request for quote, rather than a price, on their web page.