There is a mention of two exhaustive free tools for web developers over at OSTATIC – although strictly speaking I would categorise them as resources but that’s just being anal… :-). There are tonnes of useful tutorials that are very worthwhile to check out. By the way, W3 Schools has been my first point of reference for any HTML- and CSS-related questions to do with syntax and options for a long time.
However, after reading about it from a non-technical person who managed to pick up enough to start working on site development (an amazing story in itself in my opinion), I think I will add the Yahoo! User Interface Library and in particular the YUI CSS Grid Builder. It’s fantastic for doing a quick layout of websites.