Application that can fetch and update metadata of songs on iPod

I am looking for an application that can fetch and update metadata (i.e. album, artist, year, etc) of songs on iPod.

I'm sure there are a lot of applications like this out there, but i want something more specific. Mainly, it needs to somehow work with iTunes using the music on my iPod rather than working on the files on disk and then having me sync with the iPod. This is because i don't keep all my songs on my local hard drive, i only keep them on the iPod (also, some are on an external HD). I'm not sure if it is possible to do this, however i imagine it could by possible using COM to interface with iTunes (as this article suggests).

I want the metadata to be consistent throughout my library. The application should be able to find the required information for all songs (or at least, a vast majority) using some online database. At the very least, it should find the album name, artist name, year released and genre. It would also be good if it got other stuff like composers and album art. Since i at least have the title of the song, it should use this name to search for. The album and artist, if already available, should be used to narrow down the search.

Obviously, what i have mentioned is fairly specific, but i am merely saying what i want. If anyone knows of an app that can do exactly that, that would be great. But otherwise, one that comes close would probably do.