Phone Disk for iPhone:

Mount your iOS device as a drive on your computer

I actually already have a very similar tool for this on my Mac called iPhone Explorer, but Phone Disk may just offer a more elegant solution. All you have to do is install the Mac or Windows client, load it up, register it with the free official serial key (available until December 1st), and plug your iPhone in. It should then show up like any other mounted drive on your computer, and I see it popping up quite nicely in Finder (the Mac counterpart to Explorer). Since I'm jailbroken, I can also see and manipulate the contents of everything in the root ...