I've been a long time Amazon customer, but a while ago I stumbled upon The Book Depository in the UK. Not only are their prices competitive with Amazon (especially when you consider the free shipping), but I was totally sucked in by their live "Watch people shop" widget - a very cool Google maps mashup.

..although it does look a bit strange when all the book buyers in Australia seems to be based in Alice Springs;-)

Soundtrack for this post: Someone To Watch Over Me - Blossom Dearie