Visily - The easiest and most powerful wireframe software for agile teams.
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
Auto-Layout for Sketch - Responsive design for Sketch
Learn SpriteKit by Design+Code - Make a platformer iPhone game from scratch in Swift 4.