Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Nightler - Find the people. Live the night.
TechEvents.co - Tech and startup events aggregator
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Facebook Local - Discover places to go and events happening near you
Discotech - Opentable for nightlife. Tickets, guest lists, and VIP.