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Do you know about BookCrossing? It's a great site where you can register books by their ISBN number then leave them in a public place for someone to pick up and then you can track them on the Internet to see where they go.
One might argue that I'm stretching the definition of "good deed" by including this in my collection of posts about budget philanthropy and good deeds. However, I personally feel that anything that makes someone smile, or think or pause to consider the wonders of the world is a Good Deed! And this is what BookCrossers do. If you love to read and love to encourage others to read you'll love this site. It's appealing to the scavengers in us and will brighten someone's day!
I hope you'll try it out. Get some books you don't want, go register them and have fun leaving books for people to have or even just had them out to random strangers. I bet someone will smile and have a great story to tell about you later.