These days I use mostly Basscss[1] as my front-end framework. It's more than a utility layer than a framework, built on OOCSS principles. It has grid/type/colors systems, flex utils and basically everything you need for modern development.

It's a swiss knife if you are into fast prototyping and it's light. Actually, it's less than 12Kb minified and gzipped.

[1] http://www.basscss.com/