One of the most prominent features of iTunes 3 is the ability to rate your songs. This comes in handy when making those nifty Smart Playlists. I leave iTunes running constantly. When I hear a song that I want to rate, I find that it gets a little tedious to open the iTunes window, find the current song, and then drag out the number of stars I want to rate it with.
I found it much easier to rate my songs from the Dock. Point your cursor at the iTunes icon in the dock, hold Control, and click it. Remember how you can navigate and shuffle songs by doing this? Well now you can rate them too!

