Simplify Music 2Publisher:
Not even the largest-capacity iPhone can store the huge music libraries of some audiophiles. If that's the case for you, or if you're in the habit of forgetting to sync the new playlists you make to your iPhone, you might want to look into Simplify Music 2
Simplify Music 2 streams your music library from your computer straight to your iPhone via Wi-Fi or EDGE/3G. After you install a companion app on your computer (PC, Mac, or Linux), your entire music collection will be available wirelessly on your iPhone, along with search functions and playlists--it's that easy. You can also enable the app to stream music from up to 30 of your friends on your iPhone, given that they use the app as well.
One note: If you own a lot of DRM-protected files that you've bought from the iTunes Music Store, you should make sure to sync those files with your iPhone the classic way, because Simplify Music 2 can't stream those files