Gumby – Responsive CSS Framework With A Web UI Kit
Gumby is a responsive and 960px-grid CSS grid framework that can be customized to work in every resolution with almost no effort.
Its fluid-fixed layout self-optimizes the content for desktop, tablet and mobile resolutions and there is support for nested grids.
Gumby has multiple versions to download: 12 columns, 16 columns, a hybrid one or an all-in-one package. And, there are PSD files provided to ease designing with the grid.
The framework is very well-documented, has demos for all of its features and totally free to use.