Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Carrd - Simple, responsive one-page site creator.
Purecss - A set of small, responsive CSS modules that you can use in every web project.
Fluid for Sketch - Sketch plugin to make your responsive designs more efficient
Materialize CSS - A modern responsive front-end framework based on Material Design