Half Price Books has a large selection of books at a big discount, often much cheaper than half price. They have music, movies, calendars, software and some other miscellaneous things. Also, you can sell your stuff there (though you won't get very much).
Items are separated by category but beyond that it's a matter of looking through all of them to find what you want. They don't have a computerized inventory so if you're looking for something specific, you're pretty much on your own. It's much more fun to just browse around and happen to find stuff that looks interesting.