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 )

there are some things I will need to fix -like the ammount of javascript thats being loaded, expires headers and gzip compressing everything that I can. we already have these things in the sites that are live, its not a big job, maybe half a day. hopefully it will be ready for the client to add products to the database by the end of tomorrow. then it will have to wait till they have products in and it can get some fine tuning!!!