Placid - Use Placid to auto-generate images, videos & PDFs from reusable templates
Responsive Viewer - View multiple screen dimensions in one view
Imagify - Optimize and resize images online and through your CMS.
Responsinator - Designed to test responsive websites on different devices.
Landscape by Sprout Social - An interactive tool for social media image resizing
Responsive Website Test - Test any website on desktop, mobile, and tablet