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
Smartmockups - Create stunning product mockups, easily and online.
iOS Jetpack - Beautiful easy-to-use screenshot templates for App Store
AppScreens - Mockup professional-grade screenshots for your Apple and Android apps simultaneously. Designer quality, made simple.