Sortd - Rated the #1 App for Gmail
Kiwi for Gmail - Dedicated Gmail client with clean integration of Google Calendar, Contacts, Google Drive, and...
KanbanMail - A Kanban board for your emails.
Polymail Windows - Simple, powerful email on Windows
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.
Streak - Manage customers inside Gmail