Sortd - Rated the #1 App for Gmail
KanbanMail - A Kanban board for your emails.
Asana for Gmail - Turn emails into tasks without leaving your Gmail inbox
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.
Mailbutler 2.2 for Gmail - Enhance the new Gmail with a powerful, elegant add-on 💡
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.