Input.mousePosition

Description

The current mouse position in pixel coordinates. (Read Only)

The bottom-left of the screen or window is at (0, 0). The top-right of the screen or window is at (Screen.width, Screen.height).

#pragma strict
public var particle;
function Update() {
	if (Input.GetButtonDown("Fire1")) {
		var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
		if (Physics.Raycast(ray))
			Instantiate(particle, transform.position, transform.rotation);
	}
}

using UnityEngine;
using System.Collections;

public class ExampleClass : 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);
        }
    }
}