I signed up for BookCrossing a couple of years ago, and haven't done anything with it since.
If you've not heard of BC, you take hold of a book you want to get rid of, enter the details on the site, get a unique ID and then paste in a label/hand write instructions inside the cover. You then leave the book somewhere, all furtive like, and nip back to the PC to document where you put it. Some of the 'hiding' places are in plain sight, while others are more creative. (One memorable one said 'on top of the ATM at the foot of the stairs') Hopefully, whoever finds the thing then follows the instructions on the label: Go to the bookcrossing site and enter the ID, which tells everyone they picked it up.
Anyway, I just registered a brand new copy of Hal Spacejock No Free Lunch and hid it in the local park (all furtive, like).
Now I just have to remember to check the BC site from time to time, to see whether anyone admitted to picking it up ...
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)