Flow-e is a visualization layer on top of your Gmail or Outlook inbox. It provides an elegant Kanban-like workflow that's combined with the ideas behind Inbox Zero and GTD. Flow-e eliminates the need of external task management tools and transforms your inbox into a central To Do app.
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
KanbanMail - A Kanban board for your emails.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Sortd - Rated the #1 App for Gmail
MailClark - The Slack bot for external communications
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)