var particle : GameObject;
function Update () {
if (Input.GetButtonDown ("Fire1")) {
// Construct a ray from the current mouse coordinates
var ray : Ray = Camera.main.ScreenPointToRay (Input.mousePosition);
if (Physics.Raycast (ray)) {
// Create a particle if hit
Instantiate (particle, transform.position, transform.rotation);
}
}
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
public GameObject particle;
void Update() {
if (Input.GetButtonDown("Fire1")) {
Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
if (Physics.Raycast(ray))
Instantiate(particle, transform.position, transform.rotation);

}
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

public particle as GameObject

def Update():
if Input.GetButtonDown('Fire1'):
ray as Ray = Camera.main.ScreenPointToRay(Input.mousePosition)
if Physics.Raycast(ray):
Instantiate(particle, transform.position, transform.rotation)