www.bookcrossing.com - love doing this. Set your books free in the "wild" and let someone else discover them. Books are tracked on the bookcrossing website. Fun to see where they end up and a great way to find a new home for books you have read.
www.postcrossing.com The goal of this project is to allow people to receive postcards from all over the world, for free. Well, almost free! The main idea is that: if you send a postcard, you will receive one back from a random Postcrosser from somewhere in the world.