Dean, aka The Artist in Seine, always sends his work naked. And he is very creative about it, too. He also always makes sure the back and front are equally interesting (something I'm pretty bad at).
Here's some banana mail from him
And another piece (with a tiny banana) that has a piece of a computer attached to it!
Although most of my mail art is sent in a handmade envelope, I sometimes send naked mail. More about that later in the alphabet.
Have you ever sent or received an unusual item in the mail that wasn't packaged up? Did it make you or the recipient smile?