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.
MyPaint
Open Source
MyPaint is an easy-to-use painting program which works well with Wacom graphics tablets and other similar devices.
Gimp Paint Studio
Open Source
GIMP Paint Studio is a great pack of designing components like brushes, dynamics, gradient etc.
Compare Krita with alternatives
Compare Gimp Paint Studio and Krita and decide which is most suitable for you.