How to retrieve windows that have moved 'off-screen'
Chealion answered this on SuperUser.

I won't copy and paste his answer here, as he got the credit not me.

But basically, what you could do is (quoted from Chealion's answer) : 


  A second method of arranging the Windows so you can see them (if for some reason they're staying off screen) is to change to the
  application that owns the window in question, hold the Option key down
  and choose "Arrange in Front" from the Window menu. It will then
  arrange all the windows of that application in a cascade from the top
  left of the screen.