Responsive Viewer - View multiple screen dimensions in one view
Maze - Beautiful & actionable analytics for InVision prototypes
Responsinator - Designed to test responsive websites on different devices.
ProtoPie - ProtoPie is the easiest tool used to turn your UI/UX design ideas into highly interactive prototypes for mobile, desktop, web, all the way to IoT.
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.
W3C Mobile Checker - Make websites work better on mobile devices