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.
KanbanMail - A Kanban board for your emails.
TaskUnifier - TaskUnifier - Task Management Application
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.
Doit.im - Doit.