It's called postcrossing, and you can learn how to do it by Clicking Here.  Seems like it should be called postcarding, but that's just a trivial one of the many things I don't understand.   As you may know, I also do geocaching & letterboxing.  PostCrossing is an activity where you send postcards to people across the globe chosen at random and, in return, different randomly chosen people send you cards. 

These are cards I have received.  You can display cards you have received on the postcrossing website but you are restricted to images no larger than 500 pixels which, by today's monitor standards, is too small to see clearly.  Also, you are only able to show the card and I find the postmarks as interesting as the cards, so I am doing this page to display the cards I receive.  I just started this recently and don't have many cards.  Serious carders have thousands.

When you click on one of the thumbnails below you will be taken to a larger picture of the card.  Once there, you can use the navigation bar at the top of the screen to move through the larger images.