I buy a lot from BetterWorld Books because every time I buy a book, they donate a book to someone in need around the world. When I shop from them, I feel like I'm getting more bang for my buck, so to speak.
So far, BetterWorld Books has re-used/recycled over 80 million pounds of books and raised over $10 million for global literacy and local libraries. That's a cause I can get behind, and the free shipping only makes the deal sweeter.
Addendum from TA: Seconded all the way. I use BWB as well; they're a great place -- with a great business+charity idea.