PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Zeplin - Collaboration app for UI designers & frontend developers
Handoff by Marvel - Automatically turn designs into assets, specs and code
Sketch Repo - Collection of resources for anyone who uses Sketch
InVision Studio - The world’s most powerful screen design tool
Auto-Layout for Sketch - Responsive design for Sketch