I wanted to share my experience with www.paperbackswap.com. I have to say I am very impressed with their site and the wonderful books I have gotten from there.
Usually I donate books to the library or Big Brothers and Big Sisters. Then I found out out paperbackswap.com I don't quite remember how I found out, maybe from a book group I belong to. I was a bit hesitant but thought, why not? So I registered and they gave me 3 credits just for joining (I believe now they only give 2 credits) That way you can try it with no incurred expense. I ordered some books, I posted some books in my account, people ordered from me and I was on my way!! I love the wish list - basically you just add any and all books you would like to have even if they are not posted by anyone. Once someone posts the books and you are the next person in line to receive the book - it is yours!! I love that.
For the most part I have a lot of self help books. I am into that at the moment and have been for a few years now. Since I don't usually purchase books all that much, this is a great way to be able to own a book for about $2.13 a book. I get to read these books and make them available to others after I am done.
All my books are also registered with http://www.bookcrossing.com/ as well. It is a great way to keep track of where these books are.
I think the most amazing thing is that these sites are free of charge. You can pay for some additional features but for me the basics work just fine and I am happy to save my money for say ... more books? :-)