Least recently played
Via John Gruber I got to know of Attic, an iPhone app that finds the music you haven’t played in a while. From what I can read, it has all sorts of nifty features.
I personally don’t have much use for it, because my iPhone only contains music I listen to regularly (ie. the music I really like). However, for some time I’ve had a smart playlist in iTunes that does something similar:
Of course, the magic is really with iTunes, but it is interesting nonetheless. (I have music in my library I haven’t played since 2007. Until recently I believe some of it stretched all the way back to 2006.)