Krita is a professional free and open source painting program.

3D-Coat is a commercial digital sculpting program.

GIMP Paint Studio is a great pack of designing components like brushes, dynamics, gradient etc.

MyPaint is an easy-to-use painting program which works well with Wacom graphics tablets and other similar devices.

Photopea is a free online image editor, that runs without plugins. It can both read and save PSD files.

LazPaint is a raster image editor with layers. It aims at being simpler than GIMP.