Following on from my post about The Book Depository, I've found two options based in Australia that offer free shipping within Aus. These are Emporium Books* and Borders. I haven't tried either, so I can't comment further than that, but they might be worth a comparison if you're on the hunt for a particular title.
Speaking of comparisons, here is a nice little Aussie site that does a handy price comparison across numerous online booksellers, including shipping costs to Australia, and conversion into Aussie dollars: Booko.
If you're looking for rare or out of print books, try AntiQBook or Alibris. This is going outside Aus again, though, so watch those shipping costs.
Meanwhile I remain delighted that Book Depository accepts PayPal - so handy. I've placed three separate orders with them so far, including over the notorious holiday season, and I'm still very impressed.
Another site to add to the list:
CD Wow! - This site sells books, CDs, DVDs, and some cosmetics, and also offers free worldwide shipping, plus they accept PayPal. I have no personal experience with this site as yet, though.
* 01/10/2010: I'm not sure why I thought Emporium Books offered free shipping within Australia. I've just tried to purchase a book and got slapped with a $5.45 shipping fee. Maybe they've changed their policy since I last looked at the site. Annoying.
Thanks for the great links. Cheers, Simon.