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

Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform

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

Zoho Backstage - Plan, run, and analyze your event— all from one place

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

Meetup for iOS and Android - Discover groups that are all about the things you love