MultiTunes makes it quick and easy to switch between iTunes libraries from within iTunes. It will automatically restart iTunes for you, using the selected iTunes Library - and you can have as many libraries as you desire.
How is this useful? I can tell you why it is useful for me and perhaps you will find it fits what you need too. I am on a Black Macbook with a precious 80GB of internal storage. I do however spend most of my time at my desk connected to a trusty firewire drive with a much larger capacity. Using MultiTunes I'm able to have my entire library on my firewire drive while keeping a much smaller and more compact library on my laptop while I am on the go. Saving me tons of space.
I think MultiTunes is great, check it out and let me know what you think.
Alternative download link, for when/if the developer's site is down.
PUBLISHER'S NOTE: This article has been cross posted from theubergeeks.net, you may read it there if you'd prefer.