Gravity is a SASS based framework for making powerful, easily maintainable HTML5 websites. It helps you rapidly prototype your ideas and will generally make your life much easier.
There are all kinds of useful features built in, including ready made typography presets, media queries, button styles, form styling, column sizing based on variables and much more. You will need to install SASS to run the framework but, to be honest, if you're not already using SASS you're really missing out.
Gravity uses Slim as the primary template engine.