BookMooch - BookMooch is a community for exchanging used books.
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.
BookCrossing.com - It's the World's Library. It's a smart social networking site.
PenPal World - Connect with people all over the world
Random PenPals - Get a quarantine PenPal in 10 seconds
We3 - AI that finds you the most compatible people nearby