Step 1: Download the CompizConfig Settings Manager

You can do this in Ubuntu through the Software Center or Synaptic Package Manager by searching for Compiz

Step 2: Launch it

It can be accessed through the "Preferences" menu in Ubuntu.

Step 3: Enable translucent menus

Click on "Opacity, Brightness and Saturation" and click "New" under window specific settings and input the following:

(type=dropdownmenu | type=popupmenu | type=tooltip | type=unknown | name=sun-awt-X11-XWindowPeer | name=sun-awt-X11-XDialogPeer) & !(state=fullscreen)

The first for "type" tests match dropdowns, popups and tooltips. The final matches KDE menus, tooltips and other non-windows. The "name" tests match Java AWT menus, tooltips, dialogs and sub-windows. The final negated "state" test is important; it suppresses menu translucency for fullscreen apps (like games).

You can choose whichever opacity level you like, I prefer 95-97%.