960 Grid Web Design

I pretty much always partner up with a graphic designer who creates PSDs that I then use to create themes for Drupal.  This is a great tool I've sent to some of my designers to help them with layout.

http://960.gs/demo.html

P.S. Yes, I know there's a bit of controversy about how great the 960 Grid system is these days but I have to say - I still think it's a really good starting point and better than just eye-balling things in a design comp.