It is commonly addmitted that Twitter Bootstrap is a free collection of tools for creating websites and web applications.
It is the most popular project in GitHub and is used by NASA and MSNBC among others.
And everyone is getting under the spell of Bootstrap so your templates have a great future! Origin
Bootstrap was developed by Mark Otto and Jacob Thornton at Twitter as a framework to encourage consistency across internal tools. Before Bootstrap, various libraries were used for interface development, which led to inconsistencies and a high maintenance burden. According to Twitter developer Mark Otto, in face of those challenges (source wikipedia)
Twitter Bootstrap consist a lot of useful resources for template.
jQuery plugins, Complete styleguide documentation, HTML components, 12-column grid system, LESS file, Responsive Features according to resolutions and devices.Current Version is 3.3.1