What are some alternatives?
When comparing Pixel Pix and Responsive Pixel Art, you can also consider the following products
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
piCSSel-art - Draw pixel art with CSS!
Pixel Art Studio - A purpose-built drawing tool for pixel artists
Pixilart - A drawing app for delightfully retro pixel art
Pixel Art Camera - Real-time pixel art camera