NEW: Music Folder Files Not Added v2.0
Well, depending on your point of view, perhaps this is an update. Music Folder Files Not Added v2.0 is a Snow Leopard-only application (MFFNA v1.1 is still posted and will run on Tiger and Leopard) that will list the file paths of the files in your designated "iTunes Music" folder which are not in iTunes' library. Additionally, you can select a different parent folder and its contents will be compared to the iTunes library. You can then Add a selection of found files to iTunes or move them to the Trash.
This version was developed with Xcode using the new AppleScriptObjC framework and as such will only run on Snow Leopard.