Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Google Calendar - Spend less time managing your day & more time enjoying it
Geary - Geary is an email application built for GNOME 3.
Lightning Calendar - Organize your life — it’s about time!
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.
Morgen.so - All-in-one Calendar, Tasks & Scheduler. Morgen is the single hub for everything that revolves around time management.