Zen and the Art of CSS
I imagine a lot of design junkies have found this already, but you really should check out The CSS Zen Garden. The simple concept and website demonstrate the fluidity and ease of use of CSS design in a way that is truly impressive.
It’s nothing new – Thud used to do this on his website. Mac does the same on hers. It’s skinning. But the object itself – a single website that remains unchanged in terms of content while the presentation changes around it, all while discussing the subject of CSS design – makes for a powerful illustrative tool.