After mentioning Green Metropolis yesterday, I thought I'd suggest another wonderful way of passing on previously read books.
BookCrossing allows you to release them into the wild!
The idea is that each book gets given a unique registration number. Then you abandon the book somewhere it's sure to be found, such as on a train, a park bench, or in a coffee shop.
Anyone who finds the book can look it up online and let you know where it ends up, thus allowing you to track your read around the world.
You can do it for free, or turn pro with a newbie starter kit for under $20/£13.