Anima App - Design, get feedback, convert to code, publish, iterate.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Sketch Repo - Collection of resources for anyone who uses Sketch
Pixel Art Studio - A purpose-built drawing tool for pixel artists
Auto-Layout for Sketch - Responsive design for Sketch
Piskel - Piskel is a website where designers online create sprites or pixel art.