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
PartyWith - Connecting people who love to party - anytime, anywhere.
Bigtent - BigTent is a web-based organization and communication portal for groups.
Party Play - Introducing Party Play. One playlist for multiple users.