Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Mailspring - Mailspring is a mail client for Mac, Windows and Linux.

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

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.