Bootstrap is the most popular CSS Framework for developing responsive and mobile-first websites.

SCSS is a preprocessor scripting language that extends the capabilities of CSS by adding features such as variables, nesting, and mixins. It allows developers to write more efficient and maintainable CSS code, and helps to streamline the development process by reducing repetition and increasing reusability.

Gulp.js is an old but popular site building tool that automates various repetitive development tasks in web development, such as compiling Sass, minifying JavaScript, and optimizing images.