Skeleton: Responsive CSS Boilerplate
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KNACSS, a simple and lightweight CSS framework
Javascript + HTML + CSS3: Windows Metro Tiles
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Animate.css
Kanso - Free HTML/CSS Semantic Flat Buttons
Normalize.css: Make browsers render all elements more consistently.
Change Text Link Hover Effect Using CSS3 | Web Creator Box
Subtle Patterns | Free textures for your next web project.
Twitter Bootstrap Website Showcase
Web Safe Colors
CSS Font Stack :: A complete collection of web safe CSS font stacks :: Web Fonts
Responsive Grid System
Gridlock / Projects / Ben Plum
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Less Framework 4
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Wirefy | The Responsive Wireframe Framework
Sass - Syntactically Awesome Stylesheets