Really surprised nobody has mentioned Better World Books yet. Good selection of books, most of the used books seem to be former library books, and they (at least claim) to donate books to charity with every purchase.
You can get really cool books for really cheap with their bargain bin sales too.
I've also tried ThriftBooks but I got too many bad copies - one book, Lord of the Flies, which was owned by a high schooler and had writing on every single page of the book and another book which clearly had urine of some kind on it judging by how obsessed my cat was with the yellow stain on the spine. Their edition picker is also terrible, very similar to the way Amazon handles their options to where different editions are all smashed into one page.
Everyone has about the same answers here, and I agree with many, but I've got a weird one:
Super Size Me
Something about it just compels me. And it always makes me want McDonald's afterwards... I truly struggle to explain why.