Postcrossing - A postcard exchange project that invites everyone to send and receive postcards from random places in the world. For free!
BookCrossing Core - Create own BookCrossing site based on Drupal CMS.
swap-bot - Swap-bot is a online service that organizes group swaps and a community of creative individuals.
BookMooch - BookMooch is a community for exchanging used books.
Random PenPals - Get a quarantine PenPal in 10 seconds
InterPals - InterPals is a friendly community of over 5 million friends, language learners, travelers and penpals.