Gareth

This WordPress plugin displays a mini preview window on the edit post or edit page screen.
Mini Preview sits in the Page or Post tab when you are editing a page or post in WordPress.
Buttons above the preview window provide for Refresh, various display sizes, and preview in new tab.
The preview window is a fully-functional, interactive mini browser showing exactly what the page will look like, except in miniature.

Mini Preview

Displays a mini preview when a page or post is being edited.

Mathieu Thollet

Adds option for widgets to choose on which device they are displayed : mobile, desktop, or both. 
The device detection is based on wp_is_mobile function.

Widgets Device Display Option

Adds option for widgets to hide or show depending on device type : mobile, desktop, or both.

Mini Preview vs Widgets Device Display Option