Spectre.css!

Semantic empowers designers and developers by creating a shared vocabulary for UI.

Bulma is a CSS framework based on Flexbox and built with Sass

A lightweight and modular front-end frameworkfor developing fast and powerful web interfaces.ui framework full HTML / CSS, javascript component , form, theme , dropdown , responsive ...More full than bootstrap or foundation

Skeleton is a small collection of CSS & JS files that can help you rapidly develop sites that look beautiful at any size, be it a 17" laptop screen or an iPhone. Skeleton is built on three core principles:

Materialize is a modern responsive CSS framework based on Material Design by Google

A unified, HTML5-based user interface system for all popular mobile device platforms, built on the rock-solid jQuery and jQuery UI foundation. Its lightweight code is built with progressive enhancement, and has a flexible, easily themeable design.

A set of small, responsive CSS modules that you can use in every web project.

Based on Flexbox (CSS Flexible Box Layout Module), Gridlex is a very simple css grid system to quickly create modern layouts and submodules. How it works? The concept is simple: you need to wrap your .col in a .grid. That's all.

Metro UI CSS a set of styles to create a site with an interface similar to Windows 8.

Mobile Angular UI is a mobile UI framework just like Sencha Touch or jQuery Mobile. If you know Angular JS and Twitter Bootstrap you already know it!

GroundworkCSS is a 100% free and open source responsive HTML5, CSS and JavaScript Framework.

Simple bootstrap from Twitter with Metro style.