This stay-open applet watches your playing tracks and when an un-rated track starts playing displays a window with a demand it be rated.
While you could easily rate the current track from the iTunes icon in the Dock, this script interrupts you and compels you to do so.
OS X 10.6 or better only. Previous universal version available at the link below in the "Info" section.
This window will pop up when a new song begins playing.