GROUPINGS
The Groupings feature on iTunes is probably one of the lesser known and lesser used features. It is one of the options on Get Info, but the documentation on it's use is sparse to say the least, and as a result it is hardly ever used, but it is a very flexible and powerful tag.
I use it to help define my tracks a little better. I have a lot of Heavy Metal on my iPod, and most of the time I am happy to use "Metal" as a genre for playing stuff from, but sometimes I might want to listen to just Death Metal (i.e. Napalm Death), Pagan Metal (i.e. Moonsorrow) or the newer Trance Metal (like Silent Descent). Now instead of having 10 different additional genres to cover all my metal tastes, I use the Groupings tag(which is a freetext field) to act as a sub genre field (so this will have "Death", "Pagan", "Trance Metal" in it).
Now using Smart Playlists, I can listen to the music I want to the way I want to.
RESOURCES