Remotely accessing your music library is definitely a feature I didn't know I needed until I found out it were possible! One of the biggest selling points of the iPhone is in fact its music playing capabilities since it is essentially an iPod Touch with extras. However it definitely lacks in the storage department with even 16GB not cutting it to encompass most people's music libraries (hell even my iRiver which is almost 5 years old has 20GB)!
Personally, I hate having to fire up iTunes to sync my iPhone (almost as much as I hate a lowercase i in front of everything Apple) because of the need of physically using the cable. I use Appstore (read: Installous) for all the apps that I need (well not really) given that I have 30GB of data that I have not much clue of how to make use of.
Perhaps one of the main uses of iTunes (and the only reason it's on my PC) is to put songs on the iPhone. However I'm too lazy to do this everytime I add a new album or song to my library. This is where accessing your music library remotely solves all these problems at once.
Simplify Media is essentially a music streaming app, which you install on your computer (Windows, Mac, Linux) and set up a free account to either share your iTunes, Windows Media Player or WinAmp library and access it via an app for the iPhone or iPod Touch called Simplify Music 2.
This is quite nifty because unlike a few other apps that allow remote streaming, this one works on WiFi, EDGE/GPRS and 3G! It has a built-in search feature and the interface on the iPhone is quite similar to the iPod app itself. As an added bonus it has album cover, artist info as well as lyrics!
Another noteworthy feature has to be the ability to invite up to 30 people (who can use Simplify Media which is free) to access your library no matter where they are. It then pops up as a shared library in iTunes, WMP or WinAmp (based on their specified preferences). This means you can also access other people's libraries on your iPhone too!
Check out a video of the app on the iPhone as well as the desktop version for Windows, Mac and Linux: