xCSS : xCSS bases on CSS and empowers a straightforward and object-oriented workflow when developing complex style cascades.
Milligram : Milligram provides a minimal setup of styles for a fast and clean starting point. Just it! Only 2kb gzipped! It's not about a UI framework. Specially ...
InK - Interface Kit : Ink is a set of tools for quick development of web interfaces.
Ink : A Responsive Email Framework from ZURB. Quickly create responsive HTML emails that work on any device & client. Even Outlook.
UIkit : A lightweight and modular front-end framework
for developing fast and powerful web interfaces.