Anyone have a good web-site that summarizes all of the state's withholding rules, particulalry those with mandatory wtihholding?

There was a recent discussion on the subject: http://benefitslink.com/boards/index.php?showtopic=41016

This may also be helpful.
http://www.prudential.com/view/page/public/14170

This form has some useful information: http://www.statefarm.com/about/retirees/pdfs/statetax.pdf