I’m not aware of any site that demonstrates the potential of CSS more than the CSS Zen Garden.
There are currently nearly 1,000 different style sheets for the exact same page of HTML to demonstrate just how dramatically the layout can be affected by a really great designer.
I find that searching through the various layouts can be great for brainstorming, and if you were developing a commercial site for a client it might be useful to send them to this page and get some ideas of the styles they prefer before building a site for them.