Apple's GUI for MacOS X.
Main aesthetic features seem to be the general "transparency" (in addition to GUI transparency effects, that is); All elements, while still roughly rectangular, have that "drop of water" look, from which the GUI undoubtedly has got its name. General "colored glass" feeling, I guess.
Apple has been rather vigorous when trying to protect the look and feel of this GUI; Some people who have made Aqua-like GUI themes for different platforms (for example, on Linux for different X11 window managers and widget sets like Enlightenment, GTK+ and others) have got into trouble with Apple and had their works removed from public view.
(However, Microsoft got away with it...)