As much as I like to support both writers and independent booksellers, I also like bargains, and so I enjoy used book stores and I also do my share of online shopping for used books. One of my favorite sites for this is Betterworld.com which uses proceeds from book sales for worldwide literacy programs.
Today on a visit to that site I noticed the "Online Sidewalk Sale" which allows used book buyers to support local libraries. Type in your zipcode and the site will show you participating libraries near you. Then click on "browse" and the books available from that library will appear. Buy a book, support the library. Great concept.