Responsive Website Test - Test any website on desktop, mobile, and tablet
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Responsive Viewer - View multiple screen dimensions in one view
Invision - Prototyping and collaboration for design teams
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.
Zeplin - Collaboration app for UI designers & frontend developers