Facebook Events - Facebook Events enables businesses to organize gatherings, manage invitations and send notifications and reminders to their audience.

Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

DownToMeet - DownToMeet is a platform to organize, find, and attend group events.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.