Singly the most powerful and important tool in Google Analytics. If you aren't using it, you should be.

For my main website (not any of the ones in my sig), I have 25 advanced segments set up.

I track traffic by Landing Page most specifically. And advanced segmenting is really the only true way to track conversions by landing page.

So that's my favorite thing about Advanced Segments. Anybody else figured out something cool about this feature? I love to play with data!