Look At Mouse Simple

var mousePlane:GameObject; 				//The 3D plane in the scene that we project the mouse's screen location onto. This gives us a point in 3D space for the objects to look at.
var cam:Camera; 						//The camera the object(s) will be looking at.
var planeOffset:float = -1f; 			//How far away the mouse appears to be from the camera.
var mouseLocationIndicator:Transform;	//An (optional) gameobject used to show the location the object is really looking at.

function Start() {

	//If the programmer hasn't assigned a camera to cam, try to assign it the
	//Main Camera in the scene. If that fails, then let the user know how to
	//fix the problem, and destroy this script so it doesn't spam error messages
	//into the console.
	if (!cam) {
		if (!Camera.main) {
			Debug.LogError("No Main Camera found. Please either assign a camera, or tag an existing camera as 'Main Camera'.");
			Destroy(this); //Remove the script since it will only cause errors if it continues to run with no camera assigned.
			return;
		}
		cam = Camera.main;
	}

	//Parent the mousePlane to the camera's transform, so they move and rotate together.
	mousePlane.transform.parent = cam.transform;

	//Hide the plane so it doesn't show up in the scene.
	mousePlane.renderer.enabled = false;

	//Offset the plane some distance from the camera to make the objects (eg, eyes) turn more or less to look at the mouse.
	if (planeOffset<0f) {planeOffset=cam.nearClipPlane*2f;}
	mousePlane.transform.position = cam.transform.position + cam.transform.forward * planeOffset;

	//Rotate the plane so its normal points directly towards the camera (so rays cast from the camera will intersect with it).
	mousePlane.transform.up = -cam.transform.forward;
}

function Update () {

	//The point underneath the mouse on the screen covers a line in 3D. So we use a Ray
	//(a kind of straight line) intersected with a plane to find the point we want the
	//object(s) to look at.
	var ray:Ray = cam.ScreenPointToRay(Input.mousePosition);

	//Find the point of intersection between the ray and the plane.
	var hitinfo:RaycastHit;
	if (Physics.Raycast(ray,hitinfo)) {

		//If there is a point of intersection, make all the objects look at it...
		transform.LookAt(hitinfo.point);

		if (mouseLocationIndicator) {
			mouseLocationIndicator.position = hitinfo.point;
		}

	}
}