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.