Pollen
Functional CSS for the future
MVP.css - Minimalist stylesheet for HTML elements
CSS Text Shadows From CodePen - Freebie Supply
CSSPeeper
Chrome extension to see a quick overview of styles on the current page Chrome Web Store: https://chrome.google.com/webstore/detail/css-peeper/mbnbehikldjhnfehhnaidhjhoofhpehk
Pattern Lab
Create atomic design ( http://bradfrost.com/blog/post/atomic-web-design/ ) systems with Pattern Lab.
Gumby 960 Grid Responsive CSS Framework
The NEW Responsive 960 Grid CSS Framework That You’re Already Familiar With.
Golden Grid System
A folding grid for responsive design.
Responsive HTML images
… adapts your existing HTML images for mobile devices. No mark-up changes needed. Just drop it in and forget about it.
Perfect multi-column CSS liquid layouts
Skeleton
… a Beautiful Boilerplate for Responsive, Mobile-Friendly Development […] 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.