I have been a member of BookMooch since May of 2007 (about 9 months now). BookMooch is just about the coolest thing since sliced bread (of course, I never thought sliced bread was that nifty, so it doesn't take much).
BookMooch.com is a book trading site that works on a points system. 1 point = 1 book you can ask someone else to send you. You put up your inventory of books that you are willing to send to others, and you recieve 1/10th of a point for each book listed. This gives you enough points to mooch one or two books before anyone sees your books and mooches from you. You also recieve 1/10th a point for letting the system know when you have recieved books that you mooched. There is a limit to the number of "free" points you can actually use, however, since to continue to be allowed to mooch from others, you must maintain a good ratio of books sent to others as compared to books recieved.
It's not really free books, since you have to earn points by sending out books to others, but it is very cheap books, since sending a paperback by media mail is usually around $2. I have had very good experiences with BookMooch. I get books that I want, I get rid of books I don't, and although some people are slow sending out books, or have rejected my mooch requests, I haven't felt that these infrequent happenings have tarnished my overall experience. If I want a particular book, I search BookMooch first, and then half.com (because a lot of the time I can find the books I want there for $4 or $5) and then amazon.com (last because the shipping on secondhand books from various vendors is higher than that on half.com, and I usually only buy new books as impulse purchases in brick-and-mortar stores or as a last resort when there are no good affordable secondhand copies to be found).
Looking at my stats, I see that I have recieved 15 books since joining, and sent out 14.
I love getting packages in the mail, which is probably part of why I love BookMooch so much. I like the anticipation of waiting for the mail when I know something good is coming.