Postcrossing is an international postcard exchange. The main idea behind this web service is if you send a postcard, you'll receive one in return. The web service picks random senders and receivers from the pool of Postcrossing users and you'll most likely be sending to and receiving postcards from different people. Postcrossing is a good way to get stamps and postcards from all over the world, but in practice, the most active countries (USA, Finland, Germany) tend to dominate.
I stopped participating in Postcrossing in 2009 because the rate hike in international postage made this hobby a tad costly. I may return in the future.