If you are a book lover, you will really like this idea. Take books you no longer want that are cluttering up your shelves, register them on BookCrossing.com, and leave them around in public places for other people to take. That's right, just leave your books on park benches, in hotels, in coffee shops...! That's one way to get rid of your clutter!
The fun part: People can look up BookCrossing books and see what their journey has been. For example, if someone finds your book, they will see a label or handwritten note like the one posted here (image from BookCrossing's site- click it for larger version). Each book has a serial number, and the person who found it can log in and write about where they found the book and whether they liked it, etc. And if the book is special to you, you can look it up and find out what has happened to it and see how your generosity has helped others.
It's "catch & release" for books!