PenPal World - Connect with people all over the world
BookCrossing Core - Create own BookCrossing site based on Drupal CMS.
InterPals - InterPals is a friendly community of over 5 million friends, language learners, travelers and penpals.
BookMooch - BookMooch is a community for exchanging used books.
Polyglot Club - Polyglot Club is a new social network for language learners.
swap-bot - Swap-bot is a online service that organizes group swaps and a community of creative individuals.