The Gnome terminal window has the built in ability to show a transparent background. It's not true transparency, as it simply shows through the background picture, but it's close enough.

Open up a terminal window, and go to the Edit \ Current Profile menu:

Click on the Effects tab, and then check the Transparent background radio button. The slider will let you control how transparent the background is. All the way to the left is fully transparent.

I left it a little bit shaded, here's what it looks like over the default Ubuntu background:

This trick is most useful when you have a very dark or very light background.