Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Cronofy - Unified Calendar API for developers
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
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.