Avvenue allows anyone to access their iTunes library anywhere as long as they store their music on a PC and leave it awake and on:
The Avvenu Music Player™ lets you remotely listen to music you have stored (“ripped”) on your Windows® PC. Simply install the Avvenu Music Player and you can listen to your music using the web browser on any other Internet-connected Windows or Macintosh® personal computer, laptop, or Windows Mobile® 5 Smartphone.
This can be done via mobile phones as simply as via a remote computer. It’s an attractive concept, though I wonder a) what this would do to your phone bill and b) to global warming, it being one more reason to leave your PC on rather than turn it off. Rather attractively, it’s free to download.
(If memory serves, this used to be possible on a Mac, but I may be wrong.)