Postcrossing is a website dedicated to the exchange of postcards. The basis of the website is that by sending a postcard, you will receive a postcard.
More specifically, a user asks to send a postcard. They are assigned a random address, somewhere in the world, and are given an ID number to write on the postcard. They then mail it off, and soon (or not so soon), it reaches its destination. The receiver of the card then enters the ID, and the sender is then moved to the top of the list of random addresses. Thus, the exchange is not direct: if a user in Germany's postcard is received in Japan, they might in return get a postcard from Argentina. But it does take the uncertainty out of postcard exchange, because every card you send will be returned, by someone.
Postcrossing is very popular, especially in certain parts of the world. While there are members in almost every country, especially in the developed world, it seems to be especially popular in the Netherlands, Germany, and throughout Eastern Europe and the former Soviet Republics. Postcrossing is where I get most of my experience with Belarus from. The website is also very well done, and there is a number of gadgets to entertain the user, including a pie chart of countries exchanged and a graph of cards sent and received per month. The cleanness of the website and the organization of the postcrossing system are probably why it is so popular, having reached 14,000,000 cards received.
There have been two tacks in postcarding. The first, exemplified by the defunct postcardx and now taken up by sendsometing.net, is to just have people sign up, list their address and interests, and see what develops. Such sites were chaotic and inefficient, but they often had fun social sides. Postcrossing is quite regimented, and doesn't seem to lead to many long term relationships. But it is a very efficient way of obtaining postcards from around the world. I like postcrossing, despite the transitory nature of the mail I get from there.