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.
Geary - Geary is an email application built for GNOME 3.
UpTo - Life goes beyond meetings and appointments
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.