Good used bookstores combine a sort of grubby natural light (filtered either through dusty windows or great window displays) with tall shelves filled to the brim with books. Really good used bookstores have multiple levels (or rooms) and categories written by hand on something resembling construction paper. In short, they are charming in a cluttered sort of way.
Because I would rather buy books than most other things (leaving aside, of course, food, tea and beer) finding used bookstores in all the places I live is pretty essential. Book-buying can become an expensive habit if you're not careful, and if you move a lot, a used bookstore can help you decrease the weight of your boxes.
Bookslut ran a great piece this month about why used books are more satisfying than new ones, and various Toronto sites have listed their top city used bookstores. I would also like to compile a list of great bookstores (both used and new, as long as they aren't chains) from cities across Canada. Feel free to leave suggestions in the comments, or e-mail me directly, and once I have ten or so suggestions, I'll start posting them.