Anima App - Design, get feedback, convert to code, publish, iterate.
HTMLEmail.io - Responsive HTML email templates for startups & developers
Sketch Repo - Collection of resources for anyone who uses Sketch
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
MJML - The open source framework for responsive emails