here, here, and here.
Bookcrossing.com is a website where you can create a specific ID number for any book you own and then put a label inside of the book cover with that ID number. The goal is to leave that book somewhere for another person to find who will (hopefully) go to the website and log where they found the book, what they thought of it, and where they left it themselves. After that the same process continues and you can read about where your book is traveling through the world.
I've been a member for about a year now but I haven't really started participating until recently when I started selling a lot of my books. Although I haven't had much luck in getting people to log any of my books so far, I'm hoping that will change soon.
One fun way to make this a mini road trip is to go through all of the books you don't want anymore, log them on bookcrossing and then distribute them wherever you like. You could simply leave them in a library or used book store or you could pick some interesting destinations where others could find the book and later leave them someplace where people from other areas in the world might find them like an airport or popular museum.
Give the website a shot. Even if you don't get anybody replying, you still get the satisfaction of giving away an interesting book to someone that might enjoy it just as much as you did, and if you do get replies then you can live vicariously through your book as it sees areas of the world that you haven't had chance to. Who knows, you may even make it a goal to find the wandering book that was once yours!