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.
Sortd - Rated the #1 App for Gmail
Todo.ly - Todo.ly is an intuitive and easy to use online Todo list, and Task Manager. It helps you to get organized, simplify your life, and to get things done. Create todo lists, and manage them online from anywhere.
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)
Doit.im - Doit.
KanbanMail - A Kanban board for your emails.
The Hit List - Your life is complicated, but managing it doesnt need to be.