comments field in ITunes:

[David Poehlman]

The comments field is searchable so if you have audio tht has been  
contributed byseveral people to your library for instance or want to  
remember the date you added something or any other thing that ITunes  
does not already keep track of, it goes in the comments field.  you  
can put memories or notes in there for instance, "this sounds like a  
photo of a watter fall.".

Anyway, the nice thing about it is that if you use it enough, you can  
search on it and build a nice list based on that search.