The postcards I was talking about in a post yesterday are part of the Postcrossing project.
You can sign up, and initially request up to five addresses of people anywhere in the world to send a postcard to. You write the code, assigned by the website, on the card in addition to whatever else. Then you put it in the post and wait for the recipient to register yourcard upon delivery. You will be notified by email once that has happened. Your address will then be assigned to a random other Postcrosser who will send you a card, with code. You register the code on the site, and the sender get notified in turn. You get a different address every time you request one, and similarly your address is given to different people each and every time.
Personally, I use my collection of photographs (currently about 35,000) for printing on blank cards, which go in an envelope. Strictly speaking, that's not a postcard (but I don't care). My pictures are what makes my account special, so that's what I'm sticking with.
The most proficient users of Postcrossing are in Finland; other regulars are Germany, Russia, Taiwan and the USA.