Classical Archives is one of my favourite and most useful sources of recordings after iTunes. However, Classical Archives is more valuable to me because of the huge availability of MIDI files. MIDI files are very helpful for learning melody, and practicing with an accompaniment.
I am about to learn for the North Shore Singing Competitions Verdi's Aria: Di Provenza il Mar from La Traviata. Entering the composer and song title in the Classical Archives SEARCH box, I quickly find both a recording, and MIDI file for the accompaniment. I previewed the recording which I then purchased ($0.88).
More importantly, I downloaded the MIDI file - for FREE. I can use the MIDI file in iTunes, Quicktime, Garageband or Smartscore. If I want to slow, or transpose down the accompaniment whilst I learn the song, I do this easily in Smartscore or Garageband.