Responsive Viewer - View multiple screen dimensions in one view

Pixelmator - Pixelmator is an image-editing application for Mac and iPad.

Responsinator - Designed to test responsive websites on different devices.

Canva - Canva is a graphic-design platform with a drag-and-drop interface to create print or visual content while providing templates, images, and fonts. Canva makes graphic design more straightforward and accessible regardless of skill level.

Multi Screen Test - Test your site at different screen resolutions & devices.

Figma - Team-based interface design, Figma lets you collaborate on designs in real time.