Valve engineer Ben Krasnow has invented two rather odd input devices: one you control with your tongue, the other, your bum.
The tongue controller is a refitted optical mouse, housed in a brace-retainer and placed in the mouth. Sadly it's not accurate enough to be used in place of a mouse, but it could be used for "gross" (I'll say) movements such as swipe-interfaces and leafing through carousel menus.
The bum controller is a refitted set of bathroom scales which, when sat on, can turn a character left or right in a game. Again, it's not foolproof - there are issues when trying to turn more than 180 degrees because at that point you would, obviously, no longer be able to see the screen. But otherwise it seems to work fairly well.
As with most things Valve, these are experiments. They are not necessarily confirmation of Half-Life 3 and a bespoke controller that will come with it.
Krasnow's inventions could, however, be of particular use to disabled people looking for other input methods, beyond the traditional kind, with which to control games.