This really isn't a Javascript board... I'm not sure you will get an answer. I would look at the "onKeyDown", "onKeyUp", and "onKeyPress" events. You may be able to then take the value in the box and echo it out somewhere else. If you have seen a site that does this, I suggest looking at the source code. That's the advantage Javascript has over PHP... you can always look at it's source code whenever you like 🙂 (some exceptions noted, of course).
You can find some documentation and examples here.
Good Luck!