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
Geary - Geary is an email application built for GNOME 3.
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.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.