CodeCanyon - WordPress Responsive Live Preview v1.0 | 234 KB
WordPress Responsive Live Preview is a tool that helps you preview your website as it can be seen on desktop screens, mobiles and tablets. You can choose between landscape or portait mode. It's really easy, and well integrated into the WordPress UI.
Please note that this plugin won't make your site responsive, it will only show you how it's displayed on the most used devices.