Area is one of my favorite icon themes that I use under Ubuntu and fits best with dark themes. In this tutorial we will help you install and enable it under Ubuntu Oneiric or any other versions of Ubuntu.
Run the following commands from the terminal:
wget -O areao43.tar.bz2 http://goo.gl/YI4s1
sudo tar jxvf areao43.tar.bz2 -C /usr/share/icons/
Enable it now with this command:
gsettings set org.gnome.desktop.interface icon-theme 'areao43'
Or use Gnome Tweak Tool to enable Area. Here is a screenshot of it under Ubuntu Oneiric:
For more Gnome icon themes, check this list.