BookMooch: exchange books and trade them, like a book swap or book barter: http://www.bookmooch.com/
Something to add to the list of sites I never knew existed, but wholeheartedly approve of. So much so that 10 minutes after signing up I’ve got 11 books available without even digging hard into the various piles scattered around the house.
One of my colleagues uses that site and it’s great when you start. “Oh, I can get rid of that, and that, oh and I’m never going to read that again…” But then you hit a point where everyone else has creamed off the good stuff from you, and you’ve creamed off some good stuff from them, and you’re stuck.
I’d hit the earlier problem of, “Give away books? NO!”