There's an invisible workspace you can't switch to, but move windows to and from. That's the so-called scratchpad.
Unfortunately, i3 itself does not provida a convenient way to choose which window to restore from the scratchpad. Thus, this script was written.
Just run this script. It will create a list of titles of windows currently in i3's scratchpad and run a dmenu with that list, to allow you to select a window to bring back from the scratchpad.