I heard about bookcrossing.com years ago from my friend Alex.
The idea, as I understand it, is that you register a book, get a unique ID from the site, write that ID on the book, release the book "into the wild", and then (if you're lucky) you get to track that book as it moves from person to person.
A a lover of books this seems like a great idea to me!
I guess this would work best with a book you bring on holiday and leave in a hotel/resort: that would give the best change of the book ending up somewhere interesting/exotic.
Anyway today I registered 11 books which I will be releasing (to my local St Vincent de Paul shop) at the weekend.
I'll post if I ever hear about them again!