The list of the top Desktop Applications with an Open Source license.
Non-programming tasks: Documentation,
VLC of Videolan
Audio recording and editing
Audacity audio software.
Notepad++ Windows only text editor. Written in C++.
Partially taken from this list where there are a few more.
Visual Studio Code is the Open Source IDE created in Microsoft. It is written in NodeJS and its source code is available on
FreeCAD is a 3d parametric modeler It is using a lot of C++ and Python and some other technologies. Source code is available on GitHub