Yahoo! has come with lightweight CSS framework. Yahoo! is good in bringing heavy and buggy software to us but now Yahoo! is trying to keep with time and developed new PureCSS CSS framework that it targets as lightweight, easy to use. Very important point is that it is Open Source.
Paul Burgess has reviewed PureCSS Framework and it compared it in his blog post.
What Pure is good at is providing a responsive grid. This responds nicely with elements dropping below others at smaller screen sizes based on the proportion of the grid each element takes up. The grid is also incredibly easy to use. Pure CSS uses proportions of the screen size rather than number of columns to cover. A class such as pure-u-1-6 would take up 1/6th of the screen width, and pure-u-2-5 2/5ths of the screen width. Useful indeed as you can create any form of combination of fractions. Every fraction up to 1/24th is covered with a class.