Last February, I started a project called RuseMail and promptly forgot about it since I couldn't get the bindings compiled properly. This afternoon, I finished it.
The formula behind RuseMail is really really simple: FUSE + IMAP = RuseMail. (The code is not too much longer than that formula.) FUSE is a Linux tool that allows you to create filesystems in userspace, and IMAP is a mail protocol that (while internally being hideously ugly to behold) allows a mailbox to be treated like a file system. Sort of. (So far it's a read-only filesystem.)
Anyway, the code isn't much, but once again it's the idea that counts. Right? Give it a go if you're interested, but note that you'll need to install FuseFS (the Ruby FUSE bindings) for it to work.