Bootstrap is mostly used and then ZURB Foundation. Also, there are many others to save time and solve the specific purposes. However, depending on CSS frameworks is not a good idea. Learning the actual CSS is worth your time. On the other hand, a CSS framework includes a lot of things and typically more than one file including JavaScript. You really don't need all of them in a single project. And it increases the filesize & load time of your document. If you ask programmers, most of them don't like CSS frameworks. But if you want to build economy-class websites, you can use them.