AppMockUp - Create the most compelling screenshots for your app.
App Store Template - A quick App Store template for Sketch with Presets
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.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.