iPad: The iPad is by definition a touchscreen device, but there are times you want to use it but don't want to dirty up the display with your greasy fingers. MagicReader lets you read your PDFs and turn the pages by just moving your head so you don't have to use your hands at all.
It works pretty much as advertised: you just turn your head to the left or right and MagicReader will move to the next or previous page. You can even reverse the direction if you prefer. It takes a second to detect your face, but if you're actually reading and not just trying to see if it really works you'll probably stare at the page long enough for it to do that. Once it detects your face, blue stars show up in the bar at the top. When you see them, that means you can start turning your head to make the page change.
While this is a novel way of flipping pages, especially if your hands are messy, it's not the most comfortable thing in the world. It's also prone to error if you accidentally turn your head while in sight of the camera. On top of that, you have to use the MagicReader app since Apple doesn't allow you to install extensions that would allow you to navigate anything on the iPad with your face.
If you only use MagicReader to read PDFs (and it's a pretty decent PDF reader on its own), that's fine. If you're using MagicReader when your hands are dirty, there's a lot of preparation involved. But it's a free and novel app and worth a look.