RealWorld Paint - RealWorld Paint is an free portable image/photo/animation editor.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
miniPaint - Online graphics editing tool use HTML5 technologies like Canvas, Offline support, Drag and Drop.
Pixel Art Studio - A purpose-built drawing tool for pixel artists
JugiPaint - JugiPaint is a painting software with diverse tools for various types of illustration work.
Piskel - Piskel is a website where designers online create sprites or pixel art.