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.
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.
Campaign Calendar - Campaign Calendar by Bracket Labs builds a marketing calendar from your Salesforce Campaigns.