There are tons of places where you can donate books.
Public city libraries are one of the easiest: anything they don't want they sell to raise money for the facility. That's what I typically do. (Either that or I post them up on paperbackswap.com and trade 'em for books I do want...)
There are also things like booksforafrica.org. I have friends who have worked with libraries in Africa and they are always desperately short of books.
www.albany.edu/~dlafonde/Global/bookdonation.htm has a list compiled of some drop off locations, but since I have no idea where you're from I'm not sure if this is of any use.
or gotbooks.com - they'll even pick up the books, but again it's location, location, location.
Basically you can just punch 'book donation' into google, along with where you live, and you'll get plenty of hits.