Perhaps this already exists, if so, please link me to it.

The idea
Drop music into your Dropbox folder on your computer. Then open up the app which presents you with an iPod like interface for this music.

The genius
What’s great about it is, you have all of you music on your iPhone without using up disk space. Of course you may need to upgrade your Dropbox account to something greater than 2GBs to see any real value.