BookCrossing Core - Create own BookCrossing site based on Drupal CMS.
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.
BookMooch - BookMooch is a community for exchanging used books.
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.
We3 - AI that finds you the most compatible people nearby