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.

Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.

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

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