Bookcrossing.com is geocaching for books

I stumbled onto Bookcrossing.com via stumbleupon ages ago.  And the topic of geocaching came up and I remembered this awesome site.  I am hoping to get everyone I know in my neighborhood to participate because we only have 14 books in my city right now and that sounds lame to me.
Basically, you enter the book you wish to release into their site and you get a custom number.  The person who finds it then can enter that number into their system and see the journey of that book.  I think this will be a great summer activity for me and kids.  Go check it out!

From their about page:

"What is BookCrossing?

It's the World's Library. It's a smart social networking site. It's a celebration of literature and a place where books get new life. BookCrossing is the act of giving a book a unique identity so, as the book is passed from reader to reader, it can be tracked and thus connecting its readers. There are currently 1,203,362 BookCrossers and 10,321,178 books travelling throughout132 countries. Our community is changing the world and touching lives one book at a time.

What's our mission?

Very simply, we aim to connect people through books.

How do we do it?

Label. Share. Follow. Breath new life into books instead of letting your old favorites collect dust - pass them along to another reader. Our online archival and tracking system allows members to connect with other readers, journal and review literature and trade and follow their books as lives are changed through “reading and releasing”. Users are able to tag and track their individual books by marking them with BCIDs (BookCrossing Identity Numbers). Each BCID is unique to each book – once it’s registered on our site, the book can then be followed and journaled forever. BookCrossing is free to join and free to play. So don't be 'shelf'ish with your books - read and release!"