Bootstrap

Bootstrap, developed by Twitter, is a popular open source HTML/CSS/JS Framework. Designed to be intuitive, it allows a maximal productivity to any developer who knows how to use it, not only facilitating the placement of elements on web pages but also offering a great pack of tools allowing numerous tasks' automation (Nav bar, progress bars, modals...). Bootstrap's grid system allows easy and fast management of a website's responsiveness which is crucial for any serious website nowadays. Its best asset is of course its success and its community, which allows to any developer a fast answer to any question regarding it. I am myself more of a back-end developer, and Bootstrap is my best ally in this domain : Only one framework, and it's an end to any front-end problem !