Trying out Gnome themes (or for that matter, any themes) could be quite addictive.
At the same time, getting them from http:// art.gnome.org previewing the themes, untarring them is quite boring. Gnome provides a utility - gnome-art to do that for you - It lets you preview themes for applications, windows, backgrounds and splash screens, download or install them directly through a single window.
To install Gnome Art for Ubuntu/Debian
sudo apt-get install gnome-art
Start up gnome-art - gnome-art through the terminal
System > Preferences > Art Manager
It will download the theme data the first time and retrieves only the new files from then on.
You now can Preview, Download or Install directly