Gumby is a new and great responsive CSS grid framework (similar to 960px grid system) that is great for developing new projects. Why do I say this? Well, Gumby is a very customizable grid system that without any big effort can be configured to work in any resolution satisfying every need (possibilities are endless, the result is awesome). Having a self-optimizing fluid layout, it can easily fit on any desktop, tablet or mobile environment having support for nested grids also.
The responsive CSS framework is well documented, providing demos and ways to implement. The best part is that everything is free to use. Enjoy