GNOME: The Free Software Desktop Project
K Desktop Environment: Conquer Your Desktop
Windows Vista: Bring Clarity to Your World
I couldn't find one specific to the Mac, but browsing their website you can see a bunch of them, including:
"Add a new Mac to your Mac" (www.apple.com/macosx
The GNOME and KDE slogans are part of their web site's title, which might be why you never noticed it, but when searching for them in google, those are the first things you see. They each inspire different type of people: GNOME is geared toward free software advocates, and KDE is for people look to get a more power desktop (this is my opinion of what they mean, each person interprets them differently).