BookCrossing Core - Create own BookCrossing site based on Drupal CMS.
PenPal World - Connect with people all over the world
Postcrossing - A postcard exchange project that invites everyone to send and receive postcards from random places in the world. For free!
InterPals - InterPals is a friendly community of over 5 million friends, language learners, travelers and penpals.
Linear - Streamlined issue tracking for software teams
Anygram - Anygram app offers features that let you meet new people so you can make new foreign friends practice native languages to learn something new.