After having discovered Half Price Books, I will never to go to a regular bookstore again! The prices are incredible, and they have everything under the sun.
The downside is, if you want a book that has recently come out, you are not likely to find it here. Also, I sold a bunch of books back and was disappointed by the rate they bought them at.
Pros: Great selection, low price
Cons: Few 'new' books, low saleback rate