Bootstrap is a toolkit from Twitter designed to kickstart development of webapps and sites.
It includes base CSS and HTML for typography, forms, buttons, tables, grids, navigation, and more.
Nerd alert: Bootstrap is built with Less and was designed to work out of the gate with modern browsers in mind.
Interesting article concerning trends in responsive web design. A few tips for creating responsive, grid-based designs, that don't sacrifice their creative flair.