GVfs and the GOA volume monitor
A volume monitor is a D-Bus session service that uses the org.gtk.Private namespace (eg., org.gtk.Private.MTPVolumeMonitor, org.gtk.Private.UDisks2VolumeMonitor, etc.), has an entry in /usr/share/gvfs/remote-volume-monitors/, and exposes your disk partitions, iOS devices, MTP or PTP devices as volumes. These volumes eventually show up on the GTK+ places sidebar.
We wrote a volume monitor which exposes your online file storage as a volume. It listens to GNOME Online Accounts and when it notices an account that implements the GoaFiles interface it exposes it as a volume.
To unlock support for, say, Google Drive or SkyDrive we would first need to implement the protocol as a GVfs backend, and then add the GoaFiles interface to the corresponding GoaObject.