Responsinator - Designed to test responsive websites on different devices.
Dimensions Toolkit - Chrome Extension for designers to test responsive websites.
Responsive Viewer - View multiple screen dimensions in one view
One Click Test - Tool for quickly testing your design layout in one click.
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.
Flexbox Grid - Grid based on CSS3 flexbox