If I might plug a rich command shell I'm developing: http://pigshell.com (Source at https://github.com/pigshell/pigshell)

It provides

- A shell for the web. Runs in the browser, pure client-side.

- File-like abstraction for URLs and other entities exposed by web APIs

- Unix-like style of composing commands using pipes.

- Visualization using HTML

For instance, cp -r /gdrive/<username> /home will backup the contents of your Google Drive to your desktop (see http://pigshell.com/v/0.6.2/doc/gdrive.html for details). Replace /home with /dropbox/<username>, and the same command will back up GDrive to Dropbox. And so on.

While it is already useful, there is still some work, especially around hardening the file abstraction/APIs (and reams of documentation) before before it can be horizontally expanded to support a bunch more APIs and cloud stores. I am actively working on these and expect they'll take ~2 months.