While the typical PC or Mac will be able to see and access an iPhone/iPod/iPad's hard disk drive when it's plugged in for a sync, there's still a LOT of your iDevice's disk space that's not directly accessible.
Phone Disk to the rescue! This free (through December 1) Wintel/Mac program treats your iDevice like any other USB drive, letting you poke around in your data files at will — or even storing whatever your want there, iTunes be damned. All this, and no jailbreaking required! What are you waiting for, an engraved invitation?
Make sure to check out our other post on the subject: How To: Extract Data From Backups on Your iPhone