A bunch of sass based libraries to help you design your front-end, super lightweight, amazingly flexible and well thought out.
Bourbon - http://bourbon.io/ Bourbon is a library of mixins, use as much or as little as you like.
Bourbon Neat - http://neat.bourbon.io/ Neat is the grid framework which extends the Bourbon library by adding more mixins and sass functions that will allow you to create your own flexible grid system.
Bitters - http://bitters.bourbon.io/ Add some nice out of the box styles to your elements, whether they be forms, type or more.
Refills - http://refills.bourbon.io/ A component, pattern and type library which you can use to quickly scaffold your design. Need a nice tabbed UI, maybe a navigation bar, or off canvas navigation, all is covered. Refills is made to be a copy and paste drop in kind of system, you just take what you need. Each pattern/component comes with prebuilt styles but you can also use the unstyled versions which they also have available.