SOGo - SOGo is groupware server with a focus on scalability and open standards.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
Baïkal - Cal and CardDAV server
Dialpad - Switch is a cloud-based phone system built for Google Apps users.
Radicale - The Radicale Project is a complete CalDAV (calendar) and CardDAV (contact) server solution.
Aircall - Aircall is a call center software of a new generation designed for fast growing companies. Setup instantly and integrates to your CRMs