The goal of this project is to allow people to receive postcards (and stamps for some) from all over the world, for free. Well, almost free! The main idea is that: if you send a postcard, you'll receive at least one back, from a random Postcrosser from somewhere in the world.
Why? Because there are lots of people who like to receive real mail. The element of surprise of receiving postcards from different places in the world (many of which you'd probably never have heard of) can turn your mailbox into a box of surprises - and who wouldn't like that?
Check it out at http://www.postcrossing.com/