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) as GameObject;
            
        }
    }
}

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) as GameObject)