Anyone who has ever had any involvement in digital art has heard of Photoshop. It is an industry-standard for digital creatives everywhere and most people don't give any alternatives a second thought, it is Photoshop or bust.
While I am not much of a digital artist myself, I have dabbled with it in the past and occasionally I will use Photoshop to touch-up photos or create a graphic for a website.
Recently, while trawling the internet I came across some free digital painting software that has been getting a lot of attention. Krita.
To be fair to Photoshop, Krita is aimed squarely at digital painting, so cartoonists, illustrators, and concept-artists, for example, should definitely check this out. Photoshop is probably still going to excel at photo and image manipulation.
Having used both Krita and Photoshop, I can say, and it is probably obvious from the screenshot above, the user interface and the way Krita works is very similar to Photoshop, so any Photoshop veterans out there will pick this up in no time.
For total beginners, there is tons of documentation on how to use it on their website and there are also a lot of tutorials on YouTube to get you started.
You can find out more and download Krita for free from https://krita.org.