Visily - The easiest and most powerful wireframe software for agile teams.
PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Zeplin - Collaboration app for UI designers & frontend developers
Sketch Repo - Collection of resources for anyone who uses Sketch
Learn SpriteKit by Design+Code - Make a platformer iPhone game from scratch in Swift 4.
Auto-Layout for Sketch - Responsive design for Sketch