BuddyPress - WordPress social networking in a box.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Elgg - Opensource social network engine
Atmospheir - Fastest way to discover, connect and chat with your contacts
SocialEngine - Socialengine PHP: Elevating Your Online Community Experience.
Overnight - Book a last minute room with locals, in seconds.