iRate is an applescript that automatically rates iTunes via the comment field. This rating is based on 3 factors:
Days in your library
The rating you've given the song.
iTunes does a good job of letting me know what I play most often, but that 'smart' playlist is heavily biased against songs I just added...
iRate uses the factors listed above to give a dynamic rating which changes frequently. In theory, this gives an accurate representation of what your favorite songs are. In practice, it works well too.