OverGUI

public class Viewer : MonoBehaviour {
	private Rect sliderRect;

	void Awake () {
		sliderRect = new Rect((Screen.width/2) - 100, 25, 200, 75);
	}

	void OnGUI () {
		GUILayout.BeginArea (sliderRect);
		GUILayout.BeginHorizontal ();
		GUILayout.Label ("0", GUILayout.MaxWidth(10));
		keyLightStrength = GUILayout.HorizontalSlider (keyLightStrength, 0.0f, 1.0f);
		GUILayout.Label ("1", GUILayout.MaxWidth(10));
		keyLight.intensity = keyLightStrength;
		GUILayout.EndHorizontal ();
		GUILayout.Label ("Key Light Intensity: " + keyLight.intensity);
		GUILayout.EndArea ();
	}

	void  Update () {
//		if (Input.GetMouseButton(0) && sliderRect.Contains (Input.mousePosition)) {
//			Debug.Log ("Over GUI");
//		} else {
//			Debug.Log ("NOT Over GUI");

			if (Input.GetMouseButtonDown(0)) {
				previousMousePos = Input.mousePosition;
			} else {
				deltaPos = Vector3.zero;
			}

			if (Input.GetMouseButton(0)) {
				deltaPos = Input.mousePosition - previousMousePos;
				previousMousePos = Input.mousePosition;
				Vector3 newRotation = Vector3.zero;
				newRotation.x = -deltaPos.y * sensitivity;
				newRotation.y = -deltaPos.x * sensitivity;
				thisTransform.Rotate (newRotation, Space.World);
			}
//		}
	}
}