I love going to Half Price Books. They have a huge selection of books at, you guessed it, half price! You can truly get great books for cheap here, and they do even have a clearance section where books are $1-2...my dream come true!
You can also get recent books, so it's not like one of those places that only carry books from 1980 and under. You're very likely to find bestsellers here for much, much less than you could get them anywhere else.
They also sell CD's, DVD's and other non-book related items. And if you have some books you want to sell, they do give you that option here as well. Keep in mind that you won't get anywhere near what you paid for the book originally, but you'll get a fraction of the cost of the book.