I have a rubbish old computer set up at home that stores all my music, and is connected to the internet. Via INTERNET MAGIC, I'm listening to that music in an itunes-like flash page in my internet browser. It's amazing. Takes a little while to buffer the stream for each track, but then again my upload stream is slow. Only works for MP3s, too.

It's not super-simple (you might need to find out your home ip address and configure your router ports, so if that terrifies you..), but if you fancy giving it a try, you need to do the following things on the computer holding the music:

Install Apple Bonjour
http://apple.com/bonjour

Install Firefly media server
http://www.fireflymediaserver.org/

Install the fireplay add-on for firefly:
http://www.mellberg.org/FirePlay.zip

You then just browse to the IP address of your computer at home, and it streams your music to you in a webpage. You can even download song files.

Drawbacks? Have to have a computer that's turned on, and connected to the internet. (The new Winamp has a similar system too, I think, but I've never managed to get that working)