This app is bare-bones, but it does just what it needs to do: displays a simple TextEdit-like window with the lyrics of the currently playing song in iTunes.
It would be nice with some customization options for appearance (maybe something for a paid version?), font size is the only one.
My one quib with this little app is that after you have brought up the preferences window with cmd-comma there seems to be no way of closing it except with the cursor.
After Gimmesometune went out of development, I know of no other apps that can give you the lyrics in a separate window. (Although Desktoplyrics can show them on the desktop, and Cover Version can show them in an iTunes visualizer.)
This app does not find lyrics. For that I recommend the excellent freeware GetLyrical.