Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
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.
Geary - Geary is an email application built for GNOME 3.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
Slack - A messaging app for teams who see through the Earth!