Take a look at FileBrowser -
http://itunes.apple.com/gb/app/fileb...364738545?mt=8 - but don't be mislead by the headline "
Access files on remote computers" as that ignores the way that you can copy files (and folder trees) to the iPad and manage them locally, i.e. browse, move, copy, rename & "Open in ..." them for a start.
Not requiring a jail-broken pad & not too expensive either.
Strictly though, FlorenceArt is correct in that the OpenIn will copy a file to the opening app, so you do end up duplicating the files unless you delete one or the other - for me that wasn't an issue, as I use it for eBooks and just delete the book from the reader app once read, but it's a point to be aware of.