The Ubuntu default desktop uses the Gnome window manager, but there are a number of different window managers that can be installed, including KDE, Xfce and Fluxbox.

Installing Blackbox is a simple process. You will want to make sure that you have selected the alternate install repositories.

Open a terminal window and use the following command:

sudo apt-get install blackbox

Once the install is complete, log out and click on the Options button in the lower left hand corner:

You will see a menu listing all of the window managers installed on your system. Select Blackbox from the menu, and continue to log in.

The Blackbox desktop is extremely simple, and looks a lot like Fluxbox. Right clicking on the background will give you the menu:

Really simple stuff. For more information about Blackbox including configuration tips, check out the home page.