Newt Edge: The cold-blooded edge of markup and coding... has some great CSS experiments. Though they don't work in every browser, it's a great site to explore... so long as your exploration involves the Firefox web browser.
The best way to learn more about something is by doing and the CSS Playground is a terrific example of how one can benefit from experimentation. There is a lot of content on this site and you'll have to poke around to find the best or most relevant examples -- but it's a fun site that might give you an idea or two on how (or how not) to accomplish various effects.
CSS levels 2 and 3 brought with them new ways to control the elements your style rule will match beyond simple descendant selection. These include attribute selectors, adjacent siblings and child selectors. The new syntaces aren't supported by IE6, but cunning designers can still use them to add small design touches in those browsers that do support them.
"This site documents my attempts at understanding and exploring the possibilities of CSS. From standard navigation links to my more bizarre experimental techniques."
Cutting edge CSS creates some fun experiments that really push hard on known conventions & limitations.
I'm not sure why CSS Play hasn't been added to the pile yet, though I'm tempted to blame administrative oversight. Anyways, check this site out, "play" around (har har) and try to get some of this stuff to work cross-browser. Your friends and relatives will thank you for it. Well, at least I will. If that counts.
If you're interested more in ideas for different implementations than a feature that can be used by the majority of the world browsing with IE, then check out Design Meme's simple Pure CSS Pull-Down Menus article. I really appreciate how simple the effect is achieved, and it would be interesting if anyone could still keep that simplicity (and lack of jabbascript) and create a version that works in IE.
Now, whether or not pull-down menus are a good idea is another question entirely.
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