Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Zelos - A very simple app for mobile teams - tasks and chats on desktop and mobile.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Mailman 3 - Dedicated Mailman 3 installation. Unlimited lists/configuration. Free subdomain at mailman3.com. Dedicated support for list admins.
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.