I remember when GMail was by invite only, and at the time they were offering quite a larger amount of storage for Mail than anyone else so people started using their GMail drafts to store files.
That was the first time I can across such a thing.
Someone even made an extension for Windows XP that allowed you to mount GMail as a storage volume.
> GMail Drive is a Shell Namespace Extension that creates a virtual filesystem around your Google Mail account, allowing you to use Gmail as a storage medium.