Auto-Layout for Sketch - Responsive design for Sketch
Responsive Viewer - View multiple screen dimensions in one view
Responsinator - Designed to test responsive websites on different devices.
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.
Visualize browser sizes - Show the browser sizes your actual visitors use. By Polypane
Responsive Website Test - Test any website on desktop, mobile, and tablet