Postcrossing is a website that has been helping the exchange of postcards sent by postal ways since 2004.
The principle is simple : you must send a card to another user, writing a code on it to avoid giving your own adress. When he receives it, he records the code on the site. For each recorded card you sent, you can wait for a card to be sent to you by a third user... and so on.
More than 660'000 cards were exchanged since 2004 thank to Paulo Magalhaes' site.
If you don't fancy chance to receive cards, you can go on the website's forum and participate to voluntary exchange depending on countries, card's topics, etc.
Philatelicly, you receive recent stamps of Europe and North America (the technological inegality in the world is visible on the 660'000 card map).
Here are the stamps of the first "official" (i.e. with the help of the blind exchange) postcard I received from Switzerland.
The Switz Post prints the name of the stamp designer and the printer (Enschedé for the insect, Banknote Corp. for the Pro Juventute on the right).
To me, for two weeks, the pleasure to receive cards competes with the one to prepare and stamped cards.