Twitter has a new CSS framework, named Bootstrap, that they launched recently, which includes things like grids, custom form styles, tooltips and popovers, etc..
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.
It supports modern, standards-compliant browsers, but I imagine some of the niftier features would probably break down on older ones. That would be a big issue if you wanted to use Bootstrap for an ordinary website, but less so for “web apps,” where it’s more common to assume a user has a modern browser.
I haven’t tried Bootstrap yet, but you can give it a test run by hotlinking the 7kb minified CSS from their GitHub repository. Or download the LESS files from the repository if you want to customize it.
Bootstrap from Twitter [Twitter Developer Blog]