[From the maintainer: This project is no longer being maintained (and to be honest I think is too far out-of-date to be worth updating)]
Beep Edition is designed from the ground up as a mobile-first responsive theme. I created this out of the desire to make as light-weight a theme as I could, using all the tricks I know to make it a solid HTML5/CSS3 responsibly-responsive starting point. It does use SASS and Bourbon+Neat (minimal use of grids to be honest).
While very basic, it's a good starting point and lighter in code and 'stuff' than anything else I've tried.
I'm looking forward to collaborating with others on this - I'd like to develop this further with everyone's contributions. I don't want it to be an all-consuming everything theme - but I do want it to continue to serve as a solid starting point (and one that I use for rapid prototyping and 'living wireframing' with clients) for projects that can go pretty much anywhere.
Dependencies
Sass/Bourbon Bits
Sass
Bourbon+Neat
Update on 18 January, 2016
Released a D8 version! It still needs some work, but is entirely functional and a good starting point for playing around with theming in Drupal 8. Really excited to see how this develops!