Just came across this and thought of sharing with everyone: http://www.codertools.com/css_help_guide/introducing_css.aspx Though this is not an exhaustive reference, it is good for those taking the first step in CSS or HTML.

Check it out.

Update: When digging my old bookmarks, I saw this gem: http://www.smashingmagazine.com/category/coding/. See the CSS & JavaScript sections for some challenging problems and solutions. I never knew I had this wonderful site in my bookmarks collection.