Stylus, keyboard, or novel sensory techniques?

Before I start writing about programming, I would like to introduce novel input devices suitable for mobile computing appliances. Palm series of handheld devices and Pocket PCs generally have a small keyboard not useful in mobile situation. Many of such devices have a hand writing recogniser and you can actually write on a certain part of the screen normally using a stylus but how much is the success rate of the recognition? not very high I am afraid! Now the question is are there other input devices? The answer is yes!
There are a few novel sensors which allow people to communicate with mobile computing applicances. So in the coming posts I will introduce some of them in brief to give some idea how they are working.