Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Cool Cousin - Shaking up the travel world with good old human connection
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend
Triip.me - The happiest way to travel in 227 countries