PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Anima App - Design, get feedback, convert to code, publish, iterate.
Zeplin - Collaboration app for UI designers & frontend developers
Sketch Repo - Collection of resources for anyone who uses Sketch
Git Sketch Plugin - Version control for designers
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.