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.
Responsive Website Test - Test any website on desktop, mobile, and tablet
W3C Mobile Checker - Make websites work better on mobile devices