RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs
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.