Zurb Foundation Framework
Zurb Foundation claims to be the most advanced responsive design framework in the world. Personally I think they’re pretty close to that statement.
Foundation is my official favourite responsive framework. The RWD Weekly site is built on this framework, as was the Alpha version of this site. I’ve also used it to create an internal resources intranet at Squiz and it was the base that I used to create the Squiz Framework (not officially endorsed by Marketing/Brand though).
Foundation provides you with a mobile first approach to your site and is built upon Sass. They provide a helpful _settings.scss file to make global changes to the framework making upgrading to future versions incredibly simple.
I can not recommend this enough.