Graphics and image manipulation

GIMP - Gnu Image manipulation Program: 'Photoshop' alternative
Available for Linux, Windows and OSX
https://www.gimp.org/

RawTherapee: Program for viewing adjusting and processing camera raw files - acts as raw file plugin for Gimp, can be used as Lightroom alternative
Available for Linux, Windows and OSX
https://rawtherapee.com/

Xnview - XnviewMP - Xnconvert: Batch processing and image viewing/editing
Xnview is Windows only the others available on Linux, Windows and OSX
Not open source but free for non commercial use.
https://www.xnview.com/en/

Entangle: provides a graphical interface for tethered shooting. Linux only
https://entangle-photo.org/

Gscan2pdf: A graphical tool to produce PDFs from a set of images. Linux only
https://sourceforge.net/projects/gscan2pdf/files/gscan2pdf

Scribus: Open Source Desktop Publishing. Available for Linux, Windows and OSX
https://www.scribus.net/

Video editing and manipulation- Sound recording and editing

Kdenlive - Non-Linear Video Editor: Linux and Windows
Used to create and edit slideshows and edit video
https://kdenlive.org/en/

Openshot - Non-Linear Video Editor: Used to create and edit slideshows and edit video
Available for Linux, Windows and OSX
https://www.openshot.org/

Handbrake: A tool for converting video from nearly any format to a selection of modern, widely supported codecs. Available for Linux, Windows and OSX
https://handbrake.fr/

Audacity: An easy-to-use, multi-track audio editor and recorder
Available for Linux, Windows and OSX
https://www.audacityteam.org/

General - Office programs

Libreoffice: LibreOffice is a powerful office suite - Alternative to Microsoft Office
Available for Linux, Windows and OSX
https://www.libreoffice.org/