Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Facebook Events - Facebook Events enables businesses to organize gatherings, manage invitations and send notifications and reminders to their audience.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Bigtent - BigTent is a web-based organization and communication portal for groups.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.