Simplify Media allows you to stream music from your home computer (or from the iTunes libraries of 30 of your close personal friends) to your iPhone or iPod touch via WiFi. Admittedly this video is pure marketing, but the concept is cool. I installed a copy of the 1.0 alpha build from Installer.app, and gave it a spin--or at least I tried. The program requires that you have a Simplify Media account so I had to step back and grab a desktop client first. Then after painfully installing the software, signing agreements and clicking "I don't have an account yet", I finally got signed up and added my music folders. The whole process took about 20 frustrating minutes but persistence paid off. I then returned to my iPhone, signed in with my new account, and although I was able to get correct listings of my music library, none of the songs actually played back--causing "Server is not responding" errors instead.
This is one of the caveat lector instances: Boy, does this tool sound like it's going to be great but at the same time it doesn't really work as well as you might hope, at least it doesn't for me right now (some commenters have had better luck, see below). Simplify Media for iPhone is an alpha release and presumably will be finalized sometime after the official iPhone SDK ships.