I haven't tried either, but there's Cloud Connect Pro that says you can
View and manage files on your Windows, Mac, Linux, or other network services like Dropbox, MobileMe iDisk, Google Docs, or Box.net directly from the iPad [via SMB, AFP, FTP, SFTP or WEBDAV]
and FileBrowser to
[a]ccess your NAS [via CIFS/SMB] without needing to turn your computer on.
CCP is clearly more feature-rich (with e.g. integrated VNC/RDP client, printing and SMB server) but also comes with a hefty price tag ($24.99) compared to somewhat more simple FB ($3.99).
There's also an Upgrade OS X article that discusses them both.