Sortd - Rated the #1 App for Gmail
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.
KanbanMail - A Kanban board for your emails.
GoodGantt - The best Gantt chart for Trello®
Paws for Trello - The best way to use Trello on your desktop
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.