I just came across this program on Alibris and thought it was a fantastic idea. Various institutions - schools, non-profits, libraries, etc. - post wish lists for books and you can go onto alibris and buy them for donation. For anyone who isn't familiar with Alibris, it's a network of used book (and music and movie) stores who list their inventory on the website. You can get great deals on things that may be hard to find in your local area.

I'm going to be perusing wish lists and making some donations this week.

Our new Donate-A-Book program aims to fill bookshelves across the United States. Deserving institutions—libraries, schools, nonprofits—are compiling wishlists of much-needed books. And generous donors (book lovers like you) are granting wishes for bountiful books and brighter minds
Alibris Donate-A-Book Program