Zelos - A very simple app for mobile teams - tasks and chats on desktop and mobile.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
GroupServer - Email lists, with an integrated web interface, and control.
Mailman 3 - Dedicated Mailman 3 installation. Unlimited lists/configuration. Free subdomain at mailman3.com. Dedicated support for list admins.
Mailman - Mailman is free software for managing electronic mail discussion and e-newsletter lists.
Unison by Rotunda Software - Unison is a group communication tool that helps organizations connect and organize their people.