Responsive Viewer - View multiple screen dimensions in one view
Responsinator - Designed to test responsive websites on different devices.
Responsive Website Test - Test any website on desktop, mobile, and tablet
LT Browser by Lambdatest - LT Browser is a free dev friendly browser for developers and testers. You can see mobile view of website on different screen sizes and resolution on 50+ pre installed view ports. You can build, test and debug your website faster then ever.
Responsive Image Breakpoint Generator - Turn one image into the ideal set of per-device images
I Love Adaptive - See how the site is displayed on different devices