There are exceptions. I believe I threw out my copy of Hannibal out of consideration to any person who might have been tempted to read it in the future.
I highly recommend Books for Soldiers. Many American soldiers serving overseas really appreciate having something to read. I have gotten heartfelt thank-you letters from soldiers serving in Iraq that made me and my husband cry. I’ve gotten notes from the wives of the solders, thanking me for sending books overseas to their husbands. Heck, I’ve gotten Christmas cards from soldiers to whom I’ve sent books.
I’ve belonged to Bookcrossing for years, my user name is CBCD. Bookcrossing is good. Books for Solders is great. You will make a difference to someone with Books for Soldiers.
Friends of The Library will accept them.
Goodwill Industries.
Council for the Blind.
Retirement homes.
Veterans Hospitals.
Rehab Centers.
Check out the organizations in the phone book, call them before taking books to them.
I’ve been wondering about this, as I recently moved. I’ve got a box I intend to sell/give away, but what’s a good way to find out if some of the books I have are worth anything. I’ve got about 5 hardback children’s books with copyrights ranging from 1932-1965. None of them is in pristine condition. Should I bring them to a used bookseller, check on ebay, or something else?
Do you have a Freecycle Group in your area? You can’t ask for money, but people will come to you to take just about anything (even Hannibal) off your hands.
First off, I second BooksforSoldiers.com. It is a great organization. The books are really appreciated. I used to be extremely active with them and got quite a few nice notes (not that I expected them, but it was sweet.) Its also a great way to get rid of magazines you’re done with (espeically car mags, Maxim, and newsmags like Time).
Secondly, if you sell them to used book stores, don’t expect to get much money. You might get $5-10 per box if theyr’re feeling generous. It’s better than a kick in the head, but for so little money, you might as well donate them. JMHO.
I used to feel that way, too until I moved to Japan. Unlike the States, we don’t have good ways of getting rid of them. The libraries don’t want them and very few English used book stores. Living in a tiny apartment, I’ve got only a set amount of room. Anytime I buy a new book, one’s got to go.
Extra places to donate for karma points:
Public school libraries
Prison libraries
Women’s shelter
Goodwill/thrift store
Public library
Put Bookcrossing tags in them and just leave them all over the city.
Drop them off to the library! 
I have sold some books on eBay. These are not OLD books, they are mostly mysteries, hardback, from the last couple of years. I have listed about 40 of them and sold 12, which is not a great track record. (I have also been really surprised at what sold and what did not sell. A real stinker by Patricia Cornwell, not only sold but got bid way up; a perfectly marvelous one by Laura Lippman, signed by the author, didn’t sell.)
Here’s a few of what’s left: Whiteout, Ken Follett; Exile, Allan Folsom, Head Games, Eileen Dreyer, Killing of the Tinkers, Ken Bruen, The Mongol Reply, Benjamin M. Schutz (this would be a nice one for someone going through a divorce) . . .
If anybody wants any of these eBay rejects, email me.