I remember to have seen an application to monitor the traffic on my Ethernet link, in particular copying pictures from the link to the screen. I don't remember if it was a screen saver or another app...
There's a link to an X version at the bottom of the page. I don't know how well it works; it wants me to make perms changes to stuff in /dev, which I'm not really willing to do for this right now.
However, I also found this article on O'Reilly that talks a bit more about what it is, and what it does: http://www.oreillynet.com/1414.html and I want to go find a public access point somewhere and play with it..
The OS-X version is ok. And indeed you have to change these permissions... I'm not sure why, but it works. And you have to change the permissions each time you restart your Mac.