Singularity is a responsive grid system designed to be as flexible and small as possible while still providing an enormous amount of power. Singularity was originally conceived as a way to create asymmetric grids using modular scale but has evolved to support uniform column widths, asymmetric, and compound grids. The tools provided with Singularity make building these unique and complex grids a breeze. Singularity also focuses on different development styles. Your grid can be expressed as classes or it can be written to elements as needed as well as the intelligence to know where on the grid you are without having to tell it.
Singularity is open source. Fork it on Github.