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:

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.

Gramps is a program for genealogical research. It is an extremely flexible program that meets the needs of both the amateur genealogist and the serious genealogical researcher. Allows you to store, edit, and research genealogical data.
Has the ability to import GEDCOM files exported from many proprietary genealogy programs and can generate a large number of reports in many popular formats. This allows any piece of information to be entered directly into Gramps and any event to be manipulated across the entire database (in any order), helping the user research, analyze and find connections with the ability to fill gaps in relationships.

FET is a program for automatically generating timetables for schools and universities.

Typically, FET is able to solve a complex problem in a maximum of 5-20 minutes. For simpler schedules, this may take less time (a few seconds in some cases). For extremely complex schedules, this may take longer and may take several hours.

gImageReader is a simple interface for tesseract-ocr. Tesseract is probably the most accurate open source optical character recognition (OCR) software available and can recognize text in over 60 languages.

It supports automatic detection of page layout, but the user can also manually define and configure recognition areas. You can import images from disk, scanning devices, clipboard and screenshots. Supports multi-page PDF documents. Recognized text is displayed directly next to the image, and basic text editing is possible, including search/replace and removing line breaks. Spell checking of output text is also supported if appropriate dictionaries are installed.

Shotcut supports many video, audio and image formats thanks to FFmpeg.
It uses a timeline for non-linear editing of multiple tracks, which can consist of different file formats.

Integrated application development environment (IDE) in the programming languages Java, JavaFX, Ruby, Python, C++, PHP, JavaScript/HTML5, Groovy and several others. To support development for mobile platforms (J2ME), you must install the NetBeans Mobility Pack separately.

In terms of quality and capabilities, NetBeans IDE is not inferior to the best commercial (paid) integrated development environments, supporting refactoring, profiling, highlighting syntactic structures with color, autocompletion of typed structures on the fly, many predefined code templates, and more.

wxMEdit can edit files in Text/Column/Hex modes, and supports many useful features such as bookmarks, syntax highlighting, conversion to other encodings, word wrapping, word counting, and checking for updates.
It supports all major encodings (UTF8/16/32, ISO-8859-x, CP125x, KOI8, GB18030, Big5, ...) not only in Text/Column mode but also in Hexadecimal (Hex) mode.

Hydrogen is an advanced drum machine (step sequencer). The main goal is to create a professional yet simple and intuitive instrument based on drum programming patterns.

This is an alternative to commercial programs like FL Studio (R), which allow you to create music on your computer.
It includes creating melodies and beats, synthesizing and mixing sounds, and organizing samples.
You can have fun with the MIDI keyboard and much more, all in a convenient and modern interface.