You can either use GameObject.Find and GetComponent, or FindObjectOfType. If there's just one Controller script, then FindObjectOfType is more direct.

var controllerScript = FindObjectOfType(Controller);
controllerScript.someVariable = x;

It's somewhat tricky. In the HealthBarScript do the following:

  var ctrl:Controller = gameObject.GetComponent(Controller);
  // read or set any variable in Controller using ctrl:
  health = ctrl.Health;

I don't know if the var ctrl can be defined outside functions and initialised by Start(), but if it can be done it will optimize runtime performance.