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.
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.
SLOWLY - Make pen pals from your smartphone
PenPal World - Connect with people all over the world