I like Skeleton[1] for responsive design, but for that type of responsive design (where it scales with every pixel, instead of resolution milestones like Skeleton does), Ingrid[2] is a new project I saw here recently.

I can't speak for Ingrid as I haven't used it, but with Skeleton, the only thing I needed to learn was a 960-like grid framework (which you should probably know anyway.)

[1] - http://getskeleton.com

[2] - https://github.com/robertpiira/ingrid