I still need to diversify my experience with CSS frameworks but the one I've worked with the most is the YUI CSS library.
It's a four part setup - consisting of a reset, grids package, fonts setup (not entirely sure what the pros of using that are) and a base stylesheet which applies some basic formatting. You can choose which parts you want to use and they're available to use from a CDN on Yahoo! and Google.
I like the YUI library because it provides an easily switchable layout (just by changing one class or id of the container wrapper) - for example yui-t4
provides a 180px column on the right, yui-t3
provides a 300px column on the left - these templates also take ad-block sizes as well which is useful.
The base markup isn't too bad either:
<div id="doc"><!—-overall document wrapper-->
<div id="hd"><!--header / masthead--></div>
<div id="bd">
<div id="yui-main">
<div class="yui-b"><!--primary block--></div>
</div>
<div class="yui-b"><!--secondary block--></div>
</div>
<div id="ft"><!--footer--></div>
</div>