Teamup Calendar - Cloud-based calendaring using capability URL, application developed in Zurich, Switzerland.

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

ApPHP Calendar - ApPHP Calendar is a PHP calendar script that may be easily integrated and used with various PHP projects, such as schedulers, event, etc.

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

UpTo - Life goes beyond meetings and appointments

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.