US-centric - Find plenty of free or cheap cardboard boxes for your next move at U-Haul's box exchange forum. Their forum is pretty basic, divided regionally into sections for finding free used boxes or for buying and selling boxes—though according to the Cool Tools weblog, there are plenty of people on the forum happy to give away their boxes for free. For small moves, I've always found my local supermarket to be a great resource for finding free boxes, but the supplies are normally limited. There's also Craigslist or previously mentioned UsedCardboardBoxes.com, but both are normally looking to sell instead of give away (though the latter actually delivers the boxes to your doorstep, which is nice). If you've got a favourite resource for free boxes, let's hear it in the comments.
AU - obviously Craigslist is a US site - so maybe a little far away for sending cardboard boxes. :) But I imagine communities like Freecycle and other city-based communities would be a good place to start.