Zerospan is a free application that can be used to let you share your iTunes music between two computers that are not on the same local network. For example, this lets you access music on your home computer while you are at work.
This page describes the setup process in detail; note that you'll have to tweak your Address Book a bit to make it work, but that's about the extent of the complexity.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060110221341864