Previewed - Beautiful mockups & graphics for your next app
Handoff by Marvel - Automatically turn designs into assets, specs and code
Craftwork - A collection of User Interface resources made by Craftwork
Sketch Repo - Collection of resources for anyone who uses Sketch
Design Camera - Drag and drop 3D mockups
PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free