Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventjoy - The Easiest Way To Organize An Event
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
EventNinja - Transform your Eventbrite account into rich metrics
Bigtent - BigTent is a web-based organization and communication portal for groups.
MyHub iOS - Facebook Events meets Venmo