I had an (evil; don't do this) idea a while back to create a Dropbox-like program that stores all your data as binary chunks attached to draft emails spread across an arbitrary number of free email accounts.
Definitely would make an interesting learning exercise--I learned way more about SMTP/POP protocols* than I did before when I implemented demonstration SMTP/POP servers for my libgmail library before Gmail offered alternate means of access.
These days there's even the luxury of IMAP. :D
[*] About the only thing I remember now is the `HELO` and `EHLO` protocol start messages. :)