Am I Responsive↗
Review your site on 4 different viewports (desktop, laptop, tablet, mobile)
Browserstack Responsive↗
Test your site on 3000+ real devices and browsers
Emmet Re:view↗
A browser extension for displaying responsive web-pages in a side-by-side views to quickly test how it looks at different resolutions and devices
Responsivator↗
View & share how sites look at different screen sizes