As funny and true is Pencil/Paper/Brain - they are not software 
Pixelmator Pro
Krita
PIXLR
GIMP
Inkscape
Can have their place in doing things mainstream software can’t.
I’d maybe use these as additional side tools to my mainstream suite of Adobe products.
But tend to stay within the Adobe ecosystem for about 99.9% of my needs.
Other things that might be underrated and not necessarily Graphic Deisgn Software - but is learning how to create basic scripts using JavaScript.
I’ve been on this journey for about a year now, and learning how to create my own scripts has significantly improved my workflow, error detection, and general every day to day tasks are now scripted and saves me a tonne of time.
Scripts can be written for Illustartor, Photoshop and inDesign and you can even utilise BridgeTalk scripting to get out of one app into another, perform a task and get back - and it all happens under the hood in the blink of an eye.