Frysbe - Frysbe: Make global friends at random and the fling app comes up with features to send messages to strangers from every corner of the world to expand your social circle.

Postcrossing - A postcard exchange project that invites everyone to send and receive postcards from random places in the world. For free!

PenPal World - Connect with people all over the world

swap-bot - Swap-bot is a online service that organizes group swaps and a community of creative individuals.

Unbordered - Unbordered app comes up with features to help you in finding the perfect language practice and exchange partners so you can practice foreign languages right through your mobile phone.

BookCrossing.com - It's the World's Library. It's a smart social networking site.