Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Pixel Art Studio - A purpose-built drawing tool for pixel artists
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
Pixel Studio - The Pixel Art Is Always Fun
Pixilart - A drawing app for delightfully retro pixel art
Piskel - Piskel is a website where designers online create sprites or pixel art.