Responsive CSS EDITOR

This plugin simplifies the process of adding responsive css and creating breakpoints. It reduces code redundancy of declaring breakpoints. Also, it provides field to add css regardless of resposiveness under the custom css metabox. After installation, five breakpoints are created by default. You can delete those breakpoints or add new breakpoints and also order them as desired.

Plugin Documentation: http://www.wpwox.com/responsive-css-editor/