1up Events - Corporate events as a service
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Facebook Local - Discover places to go and events happening near you
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Eventjoy - The Easiest Way To Organize An Event
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.