This is the TiDDy bear, I'm still not quite sure if it's a prank or the real deal. Anyway it's really not such a bad idea.

It consists of a teddy bear you snap to your car's seat belt and place it wherever it gets uncomfortable, being possibly your shoulder or your boobs :P

For more breast comfort accessories, see the Kush or Intimia Breast pillow.

Both help support your breasts while sleeping on your side, relieving the shoulder strain from the hanging boob's weight and preventing cleavage wrinkles...

Here's a very positive review at Invest in your chest about the Kush to see that beyond looking a bit strange, it turns out to be actually helpful! :)