These are great alternatives to the more pricey well-known brands like Adobe, Autodesk, CorelDraw Graphics Suite, etc...
I have used and still use these programs in my creative moments. If you are an artist on a budget, a student, or a hobbyist who does not want to fork out a ton of money these are wonderful programs you should check out.
Free and Open-Source: Great for photo editing, graphic manipulation, and digital painting.
I have used this since high school. It has changed a lot and has always been reliable. It is probably more powerful than I think I just don't know enough about python.
Painting, comics. Has lots of brushes, and resources.
This is my go-to for my more serious paintings or comics. It comes with lots of resources and has a strong community behind it. David Revoy is a great artist and resource for this community.
Free and Open-Source: Illustrating, designing, web designing, vector imagery
I dabbled in this. I think it is a great program, I just didn't have time to learn it all and I am not a designer but I can see this being a very versatile tool for someone who is.
Free and Open- Source: 3D creation suite. modeling, rigging, animation, simulation, rendering, compositing and motion tracking, video editing, and 2D animation pipeline
When I got into this program my computer at the time could not handle it but there are lots of tutorials and resources. This is a very amazing and powerful program if you are interested in 3D art or animation.
Free and Open-Source: Video editor
I will always have this in my back pocket. It does a really nice job and has everything you need to accomplish most tasks.