Playing with software...
I was just playing with ettercap-0.6.3.1 and I do have to say for where my computing arena is this is good software. I constantly work in a switched network and and most 'traffic sniffers' won't work. The only thing I didn't like about the software was the screen size I had to set my term to. Wansn't that much to complain about though. Source code is here.

UPDATE: I did find more info about this program and the website is located at http://ettercap.sourceforge.net/