Postcrossing who are celebrating their 5th anniversary. I joined in August of 2005 shortly after they launched. I'm not sure exactly how I found their site, but have been pleased as punch that I did.
Last October I wrote a short post with one of my favorite postcards (that one was from Russia, this one is from Finland).
I go through phases where I will send lots of cards and then I will take a break for a month or two. At one time I got involved with journal swaps, themed postcards and such by logging onto their forums and searching for more specific items to trade in the mail. But now I have the magazine to keep me busy, so I have not been as active in these extra activities.
In their blog this week, they are promoting a contest where you can email a photo of yourself sending out some postcards. The winner will receive a packet of 60 cards! Well, what are you waiting for? Get your postcards, stamps and camera and get a move on!