Foundation is a very impressive and feature-rich framework for quickly prototyping + creating the production code.
The framework is responsive, works smoothly on all major browsers (including mobile) and completely open source.
It has styles for typography, links, lists, forms (with custom radio buttons, select lists and checkboxes), tables and buttons.
There is a 12 column, semi-liquid grid which works with all screen sizes and encourages building a single website for all devices.
Foundation also includes Orbit (for content sliding) and Reveal (for modal dialogs) that both are powered with jQuery.
This is definitely an exciting project for building modern websites easily and it is a good alternative to the recently launched yet popular Twitter Bootstrap.