Rating tracks in iTunes can be a real chore, and most iTunes users just don't do it. However, track ratings can be a real boon when used in smart playlists or the 'Party shuffle' mode.
AutoRate is a free (with source available) application that analyses your listening patterns and assigns track ratings accordingly. Tracks that are played frequently are given a high rating, while tracks that are listened to less frequently, or are often skipped, are given a lower rating.
No prior training is necessary -- play/skip statistics are pulled straight from an existing iTunes library. Just run this application regularly to keep ratings matched to your listening habits.
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robg adds: Rating is always a popular topic, so I figured a free app that does mass ratings based on actual listening statistics might be interesting to some...]