Sortd - Rated the #1 App for Gmail
Focuster - Ditch your todo list for an intelligent scheduler
KanbanMail - A Kanban board for your emails.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
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.
Taskbook - Like Trello but for the Terminal