eventup - Find Great Event Venues in the US
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
EventCast - EventCast is an app that helps you schedule events and easily invite participants to subscribe.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
EvenTurbo - It has a solution for any size of organization, be it a small business, local user group, regional training company or a large organization.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.