Similar functionality has been developed for the Theme Customizer API and was merged into WordPress core as of version 4.5. This plugin will automatically deactivate when you install WordPress 4.5.
When viewing the front end of the site, a « Responsive » button is added to the WordPress Toolbar. Clicking that will toggle an overlay with your website presented in various sizes for easy responsive design testing.
This can be useful both during theme development, and when writing content to verify it will be presented properly.
This plugin is a heavily modified version of Matt Kersley’s Responsive Design Testing.