BookCrossing Core - Create own BookCrossing site based on Drupal CMS.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Postcrossing - A postcard exchange project that invites everyone to send and receive postcards from random places in the world. For free!
Babble - A more personalized social network
Linear - Streamlined issue tracking for software teams
Palaround - Meet people you should be friends with.