Check out
Postcrossing.
This is a great site for people who enjoy sending/receiving postcards - I've already sent a few myself, but your address isn't assigned to someone else until one of your postcards is received and registered on the site. The variety of members is great - so far I've sent a card to a grandmother in Maryland, a middle-aged father of 3 in Austria, a young mother in Finland, and a 9 year old girl in Germany. I think there is a similar project called Bookcrossing, where you send books instead of postcards, though I think postcards are easier (and cheaper!).
The biggest concern I've heard about with this project is that they give your address to a stranger across the world somewhere, but as far as I can tell they only give your address to one person at a time, and they have a good privacy policy otherwise, so there's no worry about getting added to zillions of junk mail lists. Have a look, see what you think. It's free to join, you just pay for whatever postcards/postage you send. I'll definitely update if I run into any problems with the program, but I'm optimistic it will turn out to be a fun way to send/receive postcards around the world.