PwnTunes a new Cydia app from the makers of popular application iRealSMS allows users to use their iPhone and iPod Touch as an removable USB drive with music and image syncing functionality - without requiring any software on connecting computer. There are quite a few free iTunes alternatives but all of them require users to launch the application on connecting computer, PwnTunes on the other hand makes the iDevice detect as an removable USB drive and all you need to do is to drag-n-drop files in respective folders and it will automatically import Music/Images from there without loosing existing library on the device.

PC and Mac utility DiskAid allows you to make use of iPhone as a data storage device - but both uploading and downloading data require you have DiskAid installed on the computer, free (during beta) iPhone application USB Drive takes care of this by allowing you to use your iPhone as a true mass storage device which works right out-of-box on any computer without needing any third-party utility or driver.

You will need an jailbroken iPhone with Cydia installed to download USB Drive beta 2, also make sure you read all the instructions before using it.