In my childhood, it was the notepad widget in the Apple menu on an early Mac that caught my eye (about the same time they had the little puzzle where you slide the pieces). It was all very simple - no fiilenames or anything, just a heap of pages you could flick through. If there's a girl you like, write her name on page 9. No one will ever find it there! But still, they might...
Locked journal posts also fit into this category. Want to post about your employer without getting fired? Make it friends-only. You can also write something that only you can see (Private), but that lacks some of the excitement of posting something that you both do and don't want anyone to read. I guess you could publically post a PGP encrypted message, but who wants that on their friends page?
My latest discoveries: MD5 and Content-Description headers.
Want to post something publically but still obscure a little piece of it? Paste in that piece's MD5 hash instead. And you can use the MD5 hash as a filename for a file with the original text, so that you can later work out what you wrote.
Even a plain text email will (usually) have a MIME part (Content-Type: text/plain). And a MIME part can have a Content-Description. A header that most people will never see. Add the header with mutt and you can get all passive-aggressive on your enemies, or send one-line love letters to the person you're stalking... and then spend the rest of the week worrying about what might happen if you're boss/crush somehow stumbles across it...
I rarely have any need for either real encryption or the pseudo-cryptography I've mentioned, but I still find it intriguing.