When my local used bookstore closed a few months ago, I was heartbroken. Just about once a month, the wife and I would go, spend an hour or so and end up with a pretty extensive stack of books for very good prices. I managed to catch up on a handful of series I had missed and even found a few other things like Seth Godin's Bootstrapper's Bible.
Despite this, when they closed we floundered to find a new place to get our fix. Then my wife found BookMooch.
I wasn't too sure about it initially, but it is completely free, so I dug in and gave it a try. You simply set up an account and start adding books you want to get rid of. Each book gives you +0.1 points and you simply input the ISBN number, the site uses Amazon to get the rest of the info. Sending a book to someone gives you another point. Acknowledging that you received a book gives you another 0.1 points. On the other hand, requesting a book costs a full point and you must send one book for every five you request.
Therefore, if you add 10 books, you can request your first one. Once people start contacting you for books, you can send them off. In the US, media mail is relatively cheap, so it normally takes a few days to get a book but the costs are a few dollars tops.
So far my experience has been pleasant. I've added about 20 books in mostly SciFi and Fantasy and then I had one book requested. Then I went to my Amazon wishlist and imported some of those. Adding books and fulfilling a request immediately gave us 3 points and I made my first requests. I received all three books in just a few days and in good shape. In addition, I am now getting regular emails when my wishlist items become available… but all requests are first-come first serve, so it can be a race.
In terms of availability, there are only about 8000 members with about 65000 books listed with about a quarter of those members having sent a book. So it's not incredibly active yet, but I think making inroads into a handful of niche communities could easily launch it into the big time. Looking at my list now, I see 30% of my wishlist immediately available including the Gang of Four's Design Patterns. If I had some points at the moment, I would grab it in a second. So at the current time, you'll be able to find most Stephen King books and numerous copies of Harry Potter but newest releases and very specialized niche books simply aren't going to be there.
Overall, I think the site is great. It's simple, easy to use, and has the potential to be something even greater with a bit of growth. Adding books, requesting books, and acknowledging them is quite simple. I don't see how they're making any money on it (no ads anywhere!), but since they don't have any of my money, I'm not too concerned about it. If you're interested, give it a try. I know you have a shelf full of books sitting right next to you gathering dust. Who knows what you might end up finding…