Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
Autodesk SketchBook - SketchBook drawing and painting software provides designers, architects, and concept artists the tools to sketch ideas quickly and create beautiful illustrations.
Pixel Art Studio - A purpose-built drawing tool for pixel artists
Clip Studio Paint - The artist's software for drawing and painting.