Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Sortd - Rated the #1 App for Gmail
TaskPaper - Plain text to-do lists for Mac
KanbanMail - A Kanban board for your emails.
TickTick - TickTickis a cross-platform to-do list app & task manager helps you to get all things done and make life well organized.
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.