I think there is one reviewed in this video, [ame=http://www.youtube.com/watch?v=NNkYeayUqNM]YouTube - ‪Best Free Apps For iPhone - 25 May 2011‬‏[/ame]
You don't need any particular app to use your phone as a USB drive. All you need is i-FunBox. It is recommended to store files in the "Volumes" folder. I store whole programs in there, like iFaith, Sn0wbteeze, redsn0w, etc.