Rate your songs from the menubar.
"This little app does two things very well. First, it displays your iTunes rating (from zero to five stars) in the menu bar for the song that is currently playing, and second, I Love Stars lets you change the rating. All this is done without having to flip between programs. Just glance at the menu bar to see the current rating, or drag across the stars to change your rating."
Minimum requirement: Mac OS X 10.5