Five years on from Robotech_Master's original writeup, and http://www.bookcrossing.com is going strong. Although the site appears to be of primary interest to book geeks and those with an obsession for tracing individual items, BookCrossing provides a worldwide free resource - free in both senses, as in beer and as in speech. This appeals to impecunious students, as well as to philanthropists with large book collections they want to get rid of. The concept is:
The World is a Library
The idea is that of serendipitous discovery. Books are left in public places: on park benches, in railway carriages, on cafe tables, ready for the next interested person to pick up the book and read it. The book contains a sticker inside the front cover, which contains a unique reference number, the BCID. Visiting the website http://www.bookcrossing.com gives the option for any anonymous person to key in the BCID, see the book crossing history of where the book has been, and add a journal entry to say where the book is now.
When you've finished reading the book, it's your turn to release it into the wild. Either put it back where you found it for someone else, or indeed leave it at a new location. Besides park benches and telephone kiosks, there are preferred locations "Official Book Crossing Zones (OBCZ)", where the proprietor or publican has given his blessing, and has something of a permanent display or shelf with BookCrossing books on it.
All of the above activities can be done anonymously. There are however benefits of joining the site as a named user. Doing so will give you membership of a worldwide growing list of bibliophiles, with on-line forums, conventions, meetups, competitions, book clubs and other community involvement. The commercial side of the site sells pads of stickers, post-it notes, waterproof bags, and anything else that is needed to release books into the wild.
Read, Register, Release
It's a good idea to use books that you have read or intend to read, as there is a fourth R, which is "Review".
When you want to register a virgin book, the website generates a unique BCID for you, and you attach a sticker and complete the BCID number. Many people register their own entire book collections, in some cases for posterity - the books will all get released into the wild when they die. I can see reasons for not putting a sticker in every book, as there may be many copies people want to give to friends as permanent possessions, or indeed sell on eBay.
When registering the copy of the book, there is an opportunity to give the book a rating and submit a review. The idea is that all books released into the wild will have mini reviews available on the website.
When releasing the copy into the wild, there is the opportunity to record a location of exactly where the book has been left. This updates the status of the book to "travelling", which it will remain at, until "caught" by someone finding the book and entering the BCID.
Most book trading sites work by way of exchange. Once people want to swap a pair of books, this is transacted using the postal service.
BookMooch is slightly different as there is a virtual currency of points, hence the give and receive transactions don't take place at the same time. I'm a heavy user of BookMooch, and currently carry an inventory of >300 books available for mooching. My objective is to reduce the number of books in my possession, and so far I have given away over 200 books.
The difference with Bookcrossing is that I can pace the rate at which I deplete my inventory, by choosing when and how many books I want to release in the wild. I've started an OBCZ today, and I am looking to initiate two more OBCZs in other parts of London.