OS X/Ubuntu Users Please Take Note:

Your computer is probably advertising itself and various services connected to it via the Bonjour service. By default, this is done to any network your computer is connected to... including the ones at the coffee shops you're sitting in. Hence, I can use avahi-discover(1) to figure out that you're there and send print jobs to your printer, control your iTunes, etc. Not so good.

You probably want to turn this sort of thing off.
http://felipe-alfaro.org/blog/2005/08/19/how-to-disable-bonjour-in-apple-mac-os-x-tiger/

sudo chmod a-x /etc/init.d/avahi* in Ubuntu.