Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Groups.io - Email and web based discussion group software, includes archiving, emails, wiki, polls, file...
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Gaggle Mail - Gaggle Mail is a simple and easy to use email list manager.
DuoCircle - DuoCircle provides enterprise spam filtering and email management solutions.