You could check for keyDown events in OnGUI: EventType, Event, EventType.KeyDown, event.type

So, I guess the code would look something like this (not tested):

public void OnGUI() {
    if (Event.current.type == EventType.KeyDown) {
        KeyPressedEventHandler();
    }
}

private void KeyPressedEventHandler() {
    // do whatever you want to do when a key was pressed ;-)
}

UnityScript / JavaScript would look almost the same - just replace "public void" with "function" and "private void" with "function" and it should compile in JavaScript/UnityScript ;-)

You can also use Input.anyKeyDown()

void Update()
{
    if (Input.anyKeyDown())
    {
        DoSomething()
    }
}

NOTE: In JavaScript, the proper syntax is:

function Update () {
    if (Input.anyKeyDown) {
        DoSomething();
    }
}