Git Sketch Plugin - Version control for designers
PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Fluid for Sketch - Sketch plugin to make your responsive designs more efficient
Zeplin - Collaboration app for UI designers & frontend developers
Marvel Sketch Plugin - Rapid mobile and web prototyping with Sketch
Sketch Repo - Collection of resources for anyone who uses Sketch