Great-White-GTK is an amazing dark GTK3 theme that can be installed in Gnome-based desktop environments. It is compatible with both Unity and Gnome Shell. Here is a screenshot of Great-White-GTK under Ubuntu Oneiric:
To install and enable the Great-White-GTK theme under Ubuntu 11.10/12.04, open the terminal (Ctrl+Alt+T) and issue these commands:
wget -O great_white_gtk.zip http://goo.gl/Bg9n1
unzip great_white_gtk.zip -d ~/.themes
gsettings set org.gnome.desktop.interface gtk-theme 'Great-White-GTK'
gconftool-2 --set --type string /apps/metacity/general/theme 'Great-White-GTK'
For Ubuntu icon themes, choose one from this list. Credit for this theme goes here.