If you have multiple tabs in a Terminal window, then dragging a tab vertically rips it off and displays it in its own window, as described in this hint.

During the drag, the window is translucent as it moves around, allowing you to see what is underneath without having to change the opacity in Terminal's preferences -- the 100% opacity setting is not changed.

Now, if only a single tab is open (View -> Show Tab Bar in a new window), and you drag it vertically first, then the entire window moves around. As above, the window is translucent while dragging, allowing you to see what is underneath. This is a quick-and-easy way to see what's behind your Terminal window without moving it out of the way and then replacing it, or by permanently changing its translucency. Just pull vertically a small amount, then hold down the mouse button until you've read what you want to read.

NOTE: You must start the drag vertically. If you move the mouse horizontally first, Terminal just moves the tab in the tab bar and the window doesn't move.