Here are a couple of links to some interesting iTunes Smart Playlist ideas:
Music Only Type Playlists
Playlist for packrats
The first link starts out with building a “Music Only” smart playlist, then expands on it by building a “New and Unlistened to” list and others to get you to rate your songs, and build out more interesting lists based on those ratings.
The second link is to lists that help you find music or podcasts or videos that are eating up space that you could reclaim if you got into a pinch for disk space.