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
FB Nearby Friends - Discover which Facebook friends are nearby or on the go
Bigtent - BigTent is a web-based organization and communication portal for groups.
Facebook Local - Discover places to go and events happening near you