Pixel Art Studio - A purpose-built drawing tool for pixel artists
KiddoPaint - MS Paint in your browser (with emojis 💃😽🎨)
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
JS Paint - JSPaint is a web-based Microsoft Paint remake
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
JS Canvas - Classic MS-Paint redone as a web application