Well, here in Northumberland, there is what I think is the first used book store or something like that (or the biggest or oldest or whatever, can't remember) called Barter Books. It's gigantic and has loads of neat stuff, even antiquities like the history of Northumbria encyclopedias, etc., which cost an absolute fortune, but it's neat to see those types of things. Overall, I don't enjoy it quite as much as Half Price Books, because there seemed to be few books I was looking for, and something pretty standard like Pride and Prejudice costs much more there as a used paperback than I would ever pay in the US. I have found a few good deals there. I loved the art book selection at Half Price Books, you could get loads of great photography books for $2 - $5, and I had amassed a huge collection of those before I moved away. Alas, nothing like that at Barter Books, from what I have seen.
Having said that, I have picked up LOADS of books from the local charity shop for 10p each, and from the local market and car boot sales, where they are dirt cheap. But it's the pick-through paperback bin arrangement, so if there is something specific I am looking for, then I usually just pick it up on Amazon for cheap.