The mission of the Postcrossing Project is to allow people to receive postcards from all over the world, for free. It's main idea is simple: if you send a postcard, you will receive at least one back from a random Postcrosser from somewhere in the world. And the more you send, the more you will receive. The element of surprise of receiving postcards from different places in the world (many of which you probably have never heard of) can turn a mailbox into a box of surprises - and who wouldn't like that?