Cronofy - Unified Calendar API for developers
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
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!
Calendarix - Calendarix is a powerful and easy to use web-based calendar that runs on PHP and MySQL.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.