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.
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.