Www.960.gs is pretty good.
I dont use it on this site. nor do I use it on 4 of the sites ive done for work, but im in the middle of taking a design by the new designer at work and making our ecommerce platform work with it.
so far all I have really been doing is adding all the classes for the grid system to our existing generated divs and adding where needed and removibg where needed. once it is done I will be able to get 4 sites put out by simply replacing the images folder and using the site specific css. it feels like cheating, but the features of these 4 sites are identical. the only difference being layout ( which is all in the css )