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.
BookMooch - BookMooch is a community for exchanging used books.
Project Fixup - Meet people, not profiles
swap-bot - Swap-bot is a online service that organizes group swaps and a community of creative individuals.
Palaround - Meet people you should be friends with.