Caliber is a library for cataloging e-books. Includes library management, eBook viewer and editor, and synchronization features for eBook reading.
Allows you to convert e-books to a dozen different formats and back.
Supports synchronization with almost all e-book reading devices (for example, Kindle, Kobo, Nook).
Can automatically pull news from a range of websites/RSS feeds, format the news into an e-book and download to a connected device.

Xournal++ - Create handwritten notes, sketches and journaling with a stylus. From solving a physics problem to keeping travel diaries. It can also be useful for adding annotations to PDF files.
Supports pressure-sensitive styluses and digital pens (e.g. from devices such as Wacom tablets, Huion tablets, XP Pen, etc.)
Possibilities: