Pixel Art Studio - A purpose-built drawing tool for pixel artists
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
JS Paint - JSPaint is a web-based Microsoft Paint remake
Pixilart - A drawing app for delightfully retro pixel art
KiddoPaint - MS Paint in your browser (with emojis 💃😽🎨)
Piskel - Piskel is a website where designers online create sprites or pixel art.