Inkscape is a powerful vector editing program that is part drawing suite, part publishing software, and all-around a formiddable tool in the hands of a capable designer.
Figma lets you mock up UI components before, during, and after their development. Its cloud-based, collaborative editor (with lots of plugins) provides a lot of options for UI/UX and graphic design projects.
An open-source drawing program, Krita lets you use all manner of input devices (including drawing tablets, of course) to create sketches, illustrations, artwork, and lots of other great stuff!
FXHome's Free version of their HitFilm Express video editor is surprisingly powerful, and definitely worth checking out.
Built from the ground up using the .NET framework, Paint.NET is a superb free, extensible raster image editing suite.