iOS Design Kit - The newest library of native iOS templates
Invision - Prototyping and collaboration for design teams
Sketch Repo - Collection of resources for anyone who uses Sketch
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
SketchApp.rocks - Index of all awesome things related to Sketch.
Craftwork - A collection of User Interface resources made by Craftwork