Tablets as input devices

Hi there,

Is it possible to use a tablet with it's own operating system (android or windows or whatever) as a virtual set of buttons or controls on a wired or wirelessly connected PC?

To elaborate, I have been wondering how possible it would be to use a tablet to create additional keys for use in desktop programs.

for instance, could you write a program that would let you pair a tablet and PC to perform keystrokes? create a button on the tablet you can tap or hold and have the connected PC receive a "ctrl" key input?