At first look, filepicker.io seems to provide javascript API for developers to quickly and easily implement a file picker.

addfs.com provides users with the ability to access files kept in box, Dropbox and Google Drive in ONE window. Think of it as an online file explorer with ability to mount cloud storage (box, dropbox and Google Drive) and manage files (create, update, delete, view, and in future perhaps copy, paste or move files between clouds).