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.
Sortd - Rated the #1 App for Gmail
KanbanMail - A Kanban board for your emails.
Sunsama - Calendar and scheduling for teams
Delibr - Delibr is an end-to-end tool for product management to structure, collaborate, keep track of questions, capture decisions and conversations.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.