Responsive Viewer - View multiple screen dimensions in one view

TrackDuck - Visual feedback for web design and development.

Responsinator - Designed to test responsive websites on different devices.

Valist - Automatically created staging environments for every update with a shareable link for testing

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

Usersnap - Usersnap is a customer feedback software for SaaS companies that need to constantly improve and grow their products.