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.
KanbanMail - A Kanban board for your emails.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Sortd - Rated the #1 App for Gmail
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)
Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.