GIMP
Open Source
GIMP stands for GNU Image Manipulation Program which is an opensource image editor program supporting almost every popular image formats.
Photopea
Free
Photopea is a free online image editor, that runs without plugins. It can both read and save PSD files.
Tux Paint
Open Source
Tux Paint is a raster graphics editor (a program for creating and processing raster graphics) geared towards young children.
Krita
Open Source
Krita is a professional free and open source painting program.
Gimp Paint Studio
Open Source
GIMP Paint Studio is a great pack of designing components like brushes, dynamics, gradient etc.
Compare MyPaint with alternatives
Compare Gimp Paint Studio and MyPaint and decide which is most suitable for you.