33 Places to Hangout in the Social Networking Era

Check out this really impressive list of social networks.
33 Places to Hangout in the Social Networking Era

This entry, in particular, caught my attention:

BookCrossing - In the real world, bookcrossing is 'the practice of leaving a book in a public place to be picked up and read by others, who then do likewise.' BookCrossing aims to turn this concept virtual, and exactly that.
Best for: Book lovers