CSS UI Frameworks

http://twitter.github.io/bootstrap/
Sleek, intuitive, and powerful front-end framework for faster and easier web development.

http://foundation.zurb.com/
Foundation The most advanced responsive front-end framework in the world.

http://gumbyframework.com/
Gumby 2 is built with the power of Sass. Sass is a powerful CSS preprocessor which allows us to develop Gumby itself with much more speed — and gives you new tools to quickly customize and build on top of the Gumby Framework.

http://www.getskeleton.com/
Skeleton is a small collection of CSS 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 not a UI framework. It's a development kit that provides the most basic styles as a foundation, but is ready to adopt whatever your design or style is.

http://responsive.vermilion.com/compare.php
Responsive CSS Framework Comparison.Bootstrap,Foundation v4, Foundation v3, Skeleton.

http://metroui.org.ua/
Metro UI CSS allows to create a Web site in the style of Windows 8 quickly and without distractions on routine tasks.

http://imperavi.com/redactor/
Fantastic WYSIWYG editor on jQuery
Create word-processed text on the web using a reliable, fast and unbelievably beautiful editor.

http://aozora.github.io/bootmetro/hub.html
Metro style web framework
Simple and flexible HTML, CSS, and Javascript for web apps that wants to use the "Windows 8 App Store" style, without the need to run on Windows 8.

http://workless.ikreativ.com/
A clean & classy HTML5, CSS3 framework that helps you to get your project up and running as fast as possible and helps prevent repetitive tasks

http://reuze.me/
Reuze is a teeny-tiny front end framework that makes structuring HTML and CSS for blogs and article-heavy sites a breeze. It plays nice with other frameworks such as Foundation and Boostrap but also works equally well on it's own.

http://thisisdallas.github.io/Simple-Grid/
Simple Grid was created for developers who need a barebones grid.