Apps to Built-in Calendar

Sunbird was a standalone calendar application and also a Firefox extension developed by Mozilla, designed for managing events, tasks, and calendar synchronization. It aimed to provide a free and open-source solution for personal and professional scheduling needs.

KOrganizer is a comprehensive calendar and scheduling application, part of the integrated KDE Kontact suite. It provides robust tools for managing events, tasks, and deadlines, with extensive connectivity options and customization.

todoyu is a comprehensive web-based collaboration and project management solution designed for teams and businesses of all sizes. It integrates essential tools like task management, time tracking, resource planning, a shared calendar, and client interaction features into a single platform.

Kontact is a comprehensive personal information management suite from KDE combining email, contacts, calendar, tasks, and news in one integrated application. It offers robust synchronization options and supports various protocols.

Open365 is a comprehensive, open-source productivity suite offering cloud storage, office applications, and communication tools. Built upon a foundation of open-source technologies like KDE, Seafile, and LibreOffice, it provides a robust alternative to proprietary solutions, emphasizing privacy and control.

Lightning is a comprehensive calendar and task management add-on for Mozilla Thunderbird and Seamonkey, designed to integrate seamlessly with email. It offers robust features for scheduling, task tracking, and synchronization with various online calendar services, enhancing productivity directly within your email client.

ProtonMail is a leading secure email service utilizing end-to-end encryption and zero-access architecture to protect user privacy. Developed by scientists at CERN, it offers a robust and user-friendly platform for sending and receiving confidential communications.

Kolab Community is a comprehensive, free, and open-source groupware suite designed for collaborative work. It provides integrated email, calendar, contacts, and tasks functionalities, empowering individuals and teams to communicate and manage their data efficiently and with a strong focus on data privacy.