Getting the Windows 8 Metro look in Ubuntu 13.04 is really not all that difficult, and the end result will be a cleaner interface.
First of all, you will need to download the Windows 8 modern UI (metro) Metacity Theme from Softpedia. This is just for Metacity, which means that only the border and the title bar will be modified.
To get the most of this theme, you will also have to install another GTK 3.x Theme called FlatStudio.
In order to install the two themes, you just have to open a terminal and run the following command, in order to start a Nautilus instance with admin rights:
Copy the two themes, after extracting them from the archives, and navigate to the root of your Ubuntu 13.04 installation (not Home). Open the following folders: usr > share > themes. Paste the two themes in there and close Nautilus.
In order to activate the themes and the Windows 8 border, you must also install a small tool called GNOME tweak tool. Open a terminal and enter this command:
sudo apt-get install gnome-tweak-tool
Start the Tweak Tool app and select the FlatStudio theme where it says Gtk+ theme, and Windows 8 at Current theme.
The modifications are applied on the fly. Enjoy Ubuntu 13.04 with a Windows 8 Metro look and feel.