Axure - The most powerful way to plan, prototype and hand off to developers, all without code. Download a free trial and see why professionals choose Axure RP 9.
Responsive Viewer - View multiple screen dimensions in one view
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Responsinator - Designed to test responsive websites on different devices.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Multi Screen Test - Test your site at different screen resolutions & devices.