I am testing a service called Simplify Media, which is potentially really cool. It has not been hitch-free, but when it works, I can play any song in my home library on my iPhone or laptop, anywhere in the world where I have a connection. You can also share libraries with up to 30 friends.
It has worked fantastically on my own library, and listening to a friends’, but he had trouble getting to mine. I still don’t know why, since I was able to access mine with no problem.
Oh, yeah – it’s FREE!
jjmarcus tech solutions 
hi, i’m the friend!
just to clarify: i had no problem getting j2C’s library (and other users’) *from my iphone.* worked like a charm! i just couldn’t get to it (or any other users’) from my imac, which was also server for my library. very strange. so i’m also not sure why. the only thing i’m sure of is that every time i start up the Simpilfy Media host, if it doesn’t take up 60-80% of my CPU (imac 2.16GHz C2D) it eats up 90+% of my 3GB of memory. and usually crashes after 10 minutes. running 10.5.5 btw.
various others having similar issues on their blog. i’m sure they’re workin’ on it!