Twitter Bootstrap, of course.

From the homepage:

Sleek, intuitive, and powerful front-end framework for faster and easier web development.

Twitter Bootstrap is my first choice, if I want an easy-to-implement frontend framework starting point in a new project. I’ve used the framework for simple things like documentation, to small business websites. It is very popular, and as of this post has over 44,000 github followers on it’s github project page!

Even if you don’t plan on using it, at least go code-spelunking, it’s the best way to learn something new. I like the direction of the upcoming Bootstrap version 3, as well.