Postcrossing is an online community around an offline hobby: postcards exchange. You sign up, send postcards and receive postcards back from random places in the world. Where the postcards come from is always a surprise.
I started the project for sheer fun of receiving more postcards myself. However, these days Postcrossing reaches 192 countries and every minute there are 5 to 6 postcards being received due to it somewhere around the globe. It makes many people smile every day, and it makes me happy for doing so.