Readdle Documents is a free WebDav client for iOS which is making waves. Not only is this an excellent WebDav client but allows you to copy documents too and from almost very cloud based service. The app supports Google Drive, SkyDrive and Dropbox amongst others.
WebDav is particularly useful for accessing and saving files on regular file servers or NAS boxes. Teamed with a public IP address or dynamic IP service you can create a private cloud of your own. And without the storage restrictions of other paid and free solutions you can really go to town on storing stuff in the cloud.
I’ve found Readdle to be a top drawer iPad app and has replaced both SkyDrive and WebDav Nav+ as my go to app for accessing stored files