Book Depository is a perennial Lifehacker favourite, in part because of its "no shipping fees" policy. The site actually handles that by slightly adjusting the prices depending on your browser-detected location rather than your entered address, which means with a little browser hacking you can convince Book Depository that you're based in the UK and get an even better deal.
A poster at OzBargain picked up on a thread at Whirlpool and outlined the technique, which involves using a Firefox plugin to edit your cookies and change your "location", as well as browsing to the site via a proxy to add an additional discount code. Hit the link for the full procedure.
The technique sparked off a long thread of discussion at OzBargain and Whirlpool, with some posters arguing that it's unethical to use the technique and others noting the Book Depository will probably work out a way to block it eventually and that if it advertises "free shipping", it has to make good on the deal. I personally would feel a little uncomfortable deliberately engineering prices to what's probably a loss-making arrangement, but clearly lots of people don't. As ever, Booko is a great resource for finding the cheapest price for any given book you're after.