For several years now I have been a member of this site called bookcrossing.com. The whole point of the site is to register a book and leave it where someone will find it, with the intention of them in turn registering for the site (if they’re not already a member) and journaling their thoughts on it after they finish reading. There is even a service on the site that sends users a daily summary of “releases” in their area (maybe 3-8 daily in the San Diego area). I suppose its just like those dollars one sees floating around out there with “Where’s George” floating around on them… wheresgeorge.com has the same “tracking” aspect but I’d argue that BookCrossing is more substance-based.
While my enthusiasm for registering and releasing books has come and gone over these 3 or 4 years (I’ll go through a spurt where I register and release maybe 6 books at a time, about once a year). My books have a low “caught” ratio (3 out of 20 or so) which I think lends to my sporadic nature of participation. Had I more positive feedback, my enthusiasm would be more sustained, I think. Nevertheless, I’m still a fan of the project.
Bonus: Here’s a 2003 BBC article about it.