Radix Icons - A crisp set of 15×15 icons. Open source.
Promomatic - Easily create beautiful app store assets
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.
Screenshot Designer - Create engaging App Store screenshots