...that noone mentioned gmailfs http://richard.jones.name/google-hacks/gmail-files ystem/gmail-filesystem.html
...it's a mountable linux filesystem that uses gmail as the storage medium. It works quite well..at times google breaks it, but the authors (so far) have been pretty quick to fix as it's usually a small change, like formatting for example.
If you use this for sensitive data, id recommend encrypting the file first.
Another plus, is you can actually RUN programs from gmailfs...and yet another added bonus...when googles gmail bot searches through your mailbox to gather ad targeting data..they'll be fed a bunch of garbage..and yes you can still use it for normal emails.