Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first projects on the web. Here is the link to the official site:
Bootstrap 4 was released (1st alpha version) on 19 August 2015. One of the changes I like the most about it is that the source code of the CSS uses the SASS prepro rather than LESS (I've always preferred SASS!), but there appear to be a lot of new features that will be nice, although I haven't explored them all yet!