I belong to a group called Postcrossing. With it you send postcards to strangers and other strangers send postcards back to you. There are more than 500,000 active users right now.
Some of the members have taken the hobby of collecting postcard to a new electronic level. For instance take a look at this flickr site from Anna Bogdanovskya. She has sent and received more than 9000 postcard and they are beautifully displayed on her pages.