Twitter Bootstrap – A CSS Toolkit By Twitter
Twitter Bootstrap is a toolkit created by Twitter for kickstarting CSS when developing websites.
It is built with Less and consists of base CSS + HTML for typography, forms, buttons, tables, grids, navigation, and some more.
The grid provided is 940px wide 16-column and, for the layouts, there are solutions for both fixed + fluid ones.
Twitter Bootstrap's HTML elements have a plain-yet-elegant style where form inputs get a nice box-shadow on focus or table rows being highlighted.
The kit also includes other stuff like popovers, alerts, tabs and pagination which all use semantic XHTML.
It is actually built with Preboot, an open-source pack of mixins and variables to be used with Less and, for anyone willing to use it with Less rather than the static CSS, a detailed how-to is provided.