Bootstrap is surely the most popular free web framework on market. Realized by Twitter and published in 2011, this framework represent today a rich collection of design techniques, which equip you to develop responsive templates and web applications. Beside this, Bootstrap is high documented. You get a lot of CSS rules, components and customization possibilities on the Bootstrap documentation page. More over this, Bootstrap is today widespread and basis of much template- and framework realizations of free content management systems, like Joomla or WordPress. Just download it and create your own lightweight and responsive theme or app with Bootstrap.