Bootstrap, from Twitter

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.

Fixed layout

The default and simple 940px-wide, centered layout for just about any website or page provided by a single <div.container>.

<body>
  <div class="container">
    ...
  </div>
</body>

Fluid layout

An alternative, flexible fluid page structure with min- and max-widths and a left-hand sidebar. Great for apps and docs.

<body>
  <div class="container-fluid">
    <div class="sidebar">
      ...
    </div>
    <div class="content">
      ...
    </div>
  </div>
</body>