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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Sortd - Rated the #1 App for Gmail
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)
Things - Things is an easy to use task manager.
KanbanMail - A Kanban board for your emails.