Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Text Event Pics - Text a number to store and share photos together
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
QuickBlox - Cloud communication and data backend.
Bigtent - BigTent is a web-based organization and communication portal for groups.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.