Example Raycast with mouse click

RayCastExample.cs

using UnityEngine;
using System.Collections;

public class AiMscCrosshair : MonoBehaviour {

	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
	  if (Input.GetMouseButtonDown(0)) {
  	  Vector3 newPosition = GetScreenToWorldPos(Input.mousePosition);
  		newPosition.y = gameObject.transform.position.y;
  		GameObject newObj = Instantiate(prefabRef, newPosition, new Quaternion()) as GameObject;
	  }
	}

	Vector3 GetScreenToWorldPos (Vector3 mousePosition) {
	  // this is just an example, be careful
	  // anyways, this is more or less the way I do things so I can reuse code
		Ray ray = Camera.main.ScreenPointToRay(mousePosition);
		// I create an auxiliar temporal plane so the ray can get the position at y-axis 0.0f
		Plane plane = new Plane(Vector2.up, Vector3.zero);
		float distance = 0;
		if (plane.Raycast(ray, out distance)) {
			return ray.GetPoint(distance);
		}
		return Vector3.zero;
	}
}