This by far is the coolest app I've seen yet on the iPhone. Simplify Media let's you stream your music from your computer (iTunes) right to you iPhone/iTouch over Edge, 3G, or Wi-Fi. So you could have ZERO music stored on your iPhone and still be able to listen to your whole music library anytime, anywhere
First off: Download/install the app to your computer: (Free)
Then setup your free account and create a user name, that name is what you give to your friends so they can listen to your music.
Then just download the app for the iPhone, link below:
After it installs you enter your user name and password, and boom you (and your friends) iTunes libraries just show up ready to play.
So think about this, you share yours I share mine, and get a few other friends in the mix and you've got thousands of songs to listen to anywhere! All you have to do is once installed, click the 'invite' button and use my screen name: jayman16 (or any friends screen name) and then you give them access to your music.
So once you set up your account invite me and I will do the same, just send me an email with your screen name and I'll add you to my list.
As of right now (10:30pm Aug 14th) Simplified Media is giving away the first 100,000 of the iPhone app for FREE. After that it's only gonna be $3.99, but hey get it while it free!