PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Sketch Repo - Collection of resources for anyone who uses Sketch
Zeplin - Collaboration app for UI designers & frontend developers
Build with Sketch - Free LEGO-like kit for Sketch
Icon Slate - Icon Slate lets you easily compose, import and export desktop icons in many formats.
Auto-Layout for Sketch - Responsive design for Sketch