Responsinator - Designed to test responsive websites on different devices.
One Click Test - Tool for quickly testing your design layout in one click.
Responsive Viewer - View multiple screen dimensions in one view
W3C Mobile Checker - Make websites work better on mobile devices
Multi Screen Test - Test your site at different screen resolutions & 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.