gistfile1.cs

void Update()
{
    // attach to camera
	if (Input.GetMouseButtonDown(0)) {
		var ray = camera.ScreenPointToRay(Input.mousePosition);

		RaycastHit hit;
		if (Physics.Raycast(ray, out hit)) {
			Vector3 v = hit.point;
			// instantiate the "ping" transform at v
		}
	}
}