Sketch to App Store - Generate App Store images for all iPhone sizes from Sketch
Smartmockups - Create stunning product mockups, easily and online.
AppMockUp - Create the most compelling screenshots for your app.
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Placeit - Generate realistic product shots in seconds
iOS 8 GUI for Sketch - A Sketch template of GUI elements found in iOS 8