I use Gmail as my secret diary. Here's how:

I created a contact: username+diary@gmail.com, and a label for messages sent to that contact. I write from my main Gmail address [username@gmail.com] to [username+diary@gmail.com], and the diary entry appears instantly in my inbox. Then I file it. Within the label "diary" all my diary entries appear perfectly filed by dates. Each entry appears also in the "sent" folder, but is not a duplicate, but the same message that also appears in the inbox and then in the archive under "diary". I know because if I delete the entry from "sent", it gets deleted from the archive too.

Hope you like the hack!