Free Software for CSS
Sass
Sass stands for syntactically awesome style sheets and is a mature, stable, and powerful CSS extension language. With Sass, web designers can work smarter and faster with CSS by providing a more elegant syntax for CSS and implementing various features that are useful for creating manageable stylesheets at scale. Some of these features include adding nested rules, variables, mixins, selector inheritance, and more. Sass has been active development since 2006, and continues to provide value to the web designer community until this day. It's released free and open-source under the MIT license.
Bootstrap
A free and open-source HTML and CSS framework dedicated to responsive and mobile-first frontend web development. With Bootstrap developers can get up and running fast without writing boilerplate HTML and CSS. Bootstrap comes packed with design templates and standards for buttons, forms, navigation, typography and interface components. The framework also provides extra interactive JavaScript extensions written specifically for Bootstrap.
new.css
new.css is a classless CSS framework to write modern websites using only HTML. The framework weighs in at jut under 5kb making it an extremely performant.
Semantic UI
Semantic UI is a free and open-source framework for HTML and CSS. The framework treats words and classes as exchangeable concepts. Classes use syntax from natural languages like noun/modifier relationships, word order, and plurality to link concepts intuitively. Semantic UI provides similar benefits to that of BEM or SMACSS style guidelines.
Materialize CSS
A modern responsive front-end framework based on Material Design. With Materialize CSS you can speedup your design and development process as most of the heavy lifting is done for you with default stylings that incorporate custom components. All principles of Material Design is incorporated into the framework and can be easily utilized with components and animations that provide better feedback to the users of your websites and applications.
PureCSS
PureCSS is a free and open-source CSS framework with a minimal footprint. The entire framework and its modules weighs in at just 3.7KB minified and gzipped. With PureCSS web developers can craft responsive UI for websites that works for mobile as well as desktop. Since every line of the framework was carefully considered, you can rest assured that your page size will be kept to a minimal, and your website loads fast.
Cirrus.CSS
Cirrus.CSS is a component and utility centric SCSS framework designed for rapid prototyping of web sites and web applications. In total, Cirrus.CSS weighs in at 17.3kb GZIPPED and provides all the necessary components for creating forms, buttons, check boxes, inputs and layouts for mobile and desktop.
Charts.css
Charts.css is a modern CSS data visualization framework. It utilizes CSS utility classes to style HTML elements as charts. Allowing developers to build interactive charts and data visualization components with CSS instead of JavaScript.
Apache Cordova
Apache Cordova is a mobile application development framework that enables developers to build hybrid web applications for mobile devices using CSS3, HTML5, and JavaScript. Instead of relying on platform-specific APIs, Apache Cordova allows you to build your application once, and ship anywhere.
UIkit
UIkit A lightweight and modular front-end framework for developing fast and powerful web interfaces.