Recently a number of pre-stamped,pre-addressed postcards have been 'lost' in public spaces as a kind of experiment.

Relying only on the good will of whoever finds the postcards we want to count how many make it to their destination.

Sometimes they come back with additions, people write on who they are, or where they found the postcard. This is good.
www.foundinthepost.co.uk
www.twitter.com/foundinthepost
www.flickr.com/groups/foundinthepost