MyPaint is a free and open-source painting program developed by Martin Renold from Switzerland in 2005. It has garnered a dedicated following among digital artists, illustrators, and hobbyists alike, primarily due to its focus on simplicity and ease of use.

openCanvas (full name: GraphicEditor openCanvas) is a raster graphics program mainly used for illustration production, developed by PGN from Japan in 2000. Its functions are similar to Adobe Photoshop or Corel Painter, but it has simpler functions and a smaller file size, takes up less system resources, runs faster, and is easier to get started with.

Krita (formerly called KImageShop, Krayon) is a free, open-source and cross-platform digital painting software developed by Krita Foundation (part of the KDE Community) from Sweden. It is fully functional, capable of the entire painting process from drafting, lining, coloring to post-adjustment; can draw concept sketch, illustration, cartoon, animation, scene and 3D mapping; supports digital board, pressure sensing, anti-shake, layers, filters, color management, and other functions.

Paintstorm Studio is a professional digital painting software for a variety of platforms. As an intuitive, easy-to-use and powerful drawing tool specially designed for painters, it takes the digital painting process to a new level and makes that as comfortable as possible. It enables user to easily and quickly adjust the brushes to be suitable for their own painting style, and quickly design works in line with their own style.

Adobe seemed to have met a development bottleneck, that is why you might have found that its many popular apps such like Photoshop are completely refreshing their version number in recent years. Because since the big version introduced 3D features, I haven't seen any significant functionality or usability improvements happened on Photoshop (it even stays in the level of 10 years ago, i.e.: the functionality of Photoshop CS6 ≈ Photoshop CC 2014/2015/2017...). In other words, all things happened to Adobe apps are merely patching in some inessential places. This case, some software developers saw this chance and took advantage of its weak points. One of them is Affinity Photo - which has become one of the most popular photo editing tools now.