If you're a designer or developer you have probably heard of CSS Frameworks. Just like programming or scripting language libraries, CSS Frameworks are packages of standard compliant styling for designers to help them theme faster and more efficiently. They are quite compact and clean to use, which saves you multiple lines of custom CSS work.
Before making your decision on which framework to use I would suggest to review the design of the website and make sure the framework fits.
960 Grid System
One of the most streamlined and popular grid systems out there. It is based on a width of 960 pixel and has 2 variants of 12 columns and 16 columns.
Elastic is based on the printed layout techniques of 4 columns but allows to you create unlimited column combination and also make elastic, fixed and liquid layouts easily. It provides a set of helper classes, to accomplish hard things by pure CSS like: same-height, full-width, and vertical-center.
Yahoo's attempt to make a framework, allowing four preset page widths and ability to stack regions on top of each other. It also supports fluid designs (using %s)
A more complicated framework, Object Oriented CSS making HTML fragments and its associated CSS classes "objects." You extend the object by applying multiple classes to their root elements.
A great framework that allows you to override all its elements. It has features of typography, clean looking forms, print stylesheets, and even a mass CSS reset for browsers.