Preboot is a comprehensive and flexible collection of LESS utilities. Its original variables and mixins became the precursor to Bootstrap. Since then, it's all come full circle.
After a two-year hiatus, Preboot has been reborn with many of the LESS variable and mixin improvements from Bootstrap, along with some new enhancements, too. It's one of the lightest and most powerful tools for CSS development without any prebuilt components.
Need something more powerful? Use Bootstrap.