I found out about Postcrossing a while back from chasy (on Dreamwidth). And it's fun!
The idea is that you register, then you send postcards to random people (each time you click on "Send a postcard", you get a random user picked for you and get their address displayed), and get postcards from other random people in return! And it's kind of addictive. Especially to see things from other countries.
So far, I've sent postcards to Finland, Spain, the US, Japan, and France, and there's one for China sitting on my desk waiting to be posted, and I got a postcard back from the Netherlands!
Fun stuff. Especially, for me, writing addresses in other alphabets "properly" :) The Chinese and Japanese people only inputted ASCII versions of their addresses, so that's what I was shown, but both responded to a private message asking how to write their name and address properly.