Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Alkarta - Find happenings in your city.
TechEvents.co - Tech and startup events aggregator
Soon - Never miss something awesome
Facebook Local - Discover places to go and events happening near you
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.