Sketch to App Store - Generate App Store images for all iPhone sizes from Sketch
Radix Icons - A crisp set of 15×15 icons. Open source.
AppMockUp - Create the most compelling screenshots for your app.
MakeAppIcon - MakeAppIcon resizes and optimizes your icon designs into all formats needed for iOS and Android mobile app!
AppLaunchpad - Create free beautiful screenshots for App store/Google play
Ayecon - Ayecon is an app that allows you to easily make icons according to the required dimensions.