AppMockUp - Create the most compelling screenshots for your app.
Sketch to App Store - Generate App Store images for all iPhone sizes from Sketch
Placeit - Generate realistic product shots in seconds
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Smartmockups - Create stunning product mockups, easily and online.
DaVinci Apps - Сreate customized screenshots for your app in just 3 steps