File manipulation in the Metro interface -- any touch-centric interface, for that matter -- is notoriously difficult. Dropbox puts a brave face on a difficult situation. With an extensive selection of file previewers, hooks to major apps (for example, you can open a DOCX file directly from Dropbox), file sharing via the Share charm, and searching with the Search charm, Dropbox is a worthy contender in the Metro sphere to Microsoft’s SkyDrive. Bonus: Dropbox doesn’t require a Microsoft account.
There’s a script you can use to put DropBox on Office 2013’s Open and Save menus.
Dropbox: 2GB free; more space from $10/month; team options