Responsive Viewer - View multiple screen dimensions in one view
Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
Responsinator - Designed to test responsive websites on different devices.
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Responsive Website Test - Test any website on desktop, mobile, and tablet
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.