So you are browsing through your iTunes library of one-hundred-gazillion songs and twenty-trillion playlists, and you get to a certain song. How do you find what playlists it is in?

In the later versions of iTunes, just right-click (control-click) on the song and choose "Show in Playlist". This will bring up a list of all the playlists that contain that song. Neat.

To work the other way around, and with multiple playlists, you can create smart playlists to find songs that are in a number of playlists you specify. For instance, create a new smart playlist and set it to match all the conditions, which are something like; Playlist is 60s tunes, Playlist is Happy Mood, etc.