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Sortd - Rated the #1 App for Gmail
Pocket - When you find something you want to view later, put it in Pocket.
KanbanMail - A Kanban board for your emails.
Flow-e - Turn your Gmail or Office365 inbox to a Visual Task Board.
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.
Diigo - Diigo is a powerful research tool and a knowledge-sharing community