Definitely what Oli (EDIT: and emilpaun and cam) said.
Also...
http://www.kevinleitch.co.uk/projectnew/index.php
http://css.maxdesign.com.au/ (I use this
all the time)
http://www.alistapart.com
CSS Layout:
http://glish.com/css/ - more CSS layouts.
Link Collections:
http://www.dezwozhere.com/links.html
http://www.alvit.de/web-dev/
http://www.alvit.de/handbook/
Web Standards Solutions - by Dan Cederholm of
www.simplebits.com is an absolute god-send. This is the book that really moved me from tables to CSS. Everyone should own it. Best CSS book I've read, by far. Cheap, too, at $23 (USD). It has everything in it - lists, navigation, layouts, text styles, table styling and so on and so forth.
What he stresses as far as (X)HTML is concerned - is semantics. While it's not so important to stress over the minutia of semantics, it is important to recognize their value and use them as best you can within the framework of (X)HTML. For more on that, I recommend his
SimpleQuiz, which is basically a discussion of how best to mark up some content in a given situation. Quite eye-opening.
I credit him with my move to web standards.
Hope this helps!