Graphics Design
Includes a plethora of resources for you to be able to design and create visual aids and illustrations.
Awesome Design (by gztchan)
An awesome list of design-related resources for you to explore more of it, if you're looking for more.
Canva
Why not create your own creative designs.
Coolors
An intuitive color scheme generator.
draw.io
A web-based online diagram and flowchart maker software.
Personal comment:
I recommend this a lot if you want to design some visual aid. Also comes with desktop versions though I don't really recommend it as it's mostly the same except you run it in the desktop.
Figma
Basically, a FREE web-based version of Adobe XD.
Personal comment:
Recommended for designing and prototyping apps.
GIMP
A free and open source photo editor similar to Adobe's Photoshop. You can view its source code right here.
Personal comment:
Yup, this is basically a viable free replacement for Photoshop
ImageMagick
It's a free and open source image manipulation program similar to Inkscape. Unlike the aforementioned tool, this program offers the approach of doing the process without a GUI, enabling you to make it faster (if you've got the hang of it).
Inkscape
A free and open source vector illustration tool similar to Adobe's Illustrator. If you're interested in seeing the source code for yourself, you can view it here.
Personal comment:
Yup, this is basically a viable free replacement for Illustrator.
Krita
A professional free and open source program for professionals, I think. It is available on the major desktop platforms: Windows, Mac OS, and Linux. If you want to visit the repo yourself, you can visit it from their GitHub repo.
Scribus
A cross-platform free and open source page layout program. Mainly used for posters, magazine production, book covers, pamphlets, and more.
Personal comment:
My go-to color scheme generator.