I have just discovered for myself a clean and quite finished GTK theme for my Ubuntu desktop. It is called the "New Wave". To use the theme, just invoke System>Preferences>Appearance and select the "New Wave" theme, as usual.
To make the theme usable, you need to switch to the "New Wave Dark Menus" variant - before you confirm the "Appearance Preferrences" dialog, click "Configure" and select the "New Wave Dark Menus" in the "Controls" tab.
This is what you get:
Here are some more previews (the theme suits quite well even to the NetBeans IDE):
