What are some alternatives?
When comparing Rapid Responsive Design and Balsamiq, you can also consider the following products
Responsive Viewer - View multiple screen dimensions in one view
Invision - Prototyping and collaboration for design teams
Responsive Design Checker - Check how responsive is your website across device sizes
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Auto-Layout for Sketch - Responsive design for Sketch
Zeplin - Collaboration app for UI designers & frontend developers