Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Noisli - Noisli is a fantastic background noise and color generator for working and relaxing. Online soothing ambient sounds like White noise, Rain and Coffee Shop.
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.
Noizio - Feeling that the noise of the big city is too much for you to bear?
Geary - Geary is an email application built for GNOME 3.
MyNoise - Custom shaped online noise machines. Many Many generic sounds