* RemoteStorage.js[1] (they plan on adding support for Dropbox, Drive, etc.)
* CloudRail[2] (nonfree source)
* CloudDock[3] (proxy service)
* byoFS.js[4] (encrypted cloud storage, I maintain this)
[1]: https://github.com/remotestorage/remotestorage.js
[2]: http://cloudrail.com/
[3]: http://clouddock.co/
[4]: https://github.com/diafygi/byoFS